V2: with parameters instead of mixins

@AlbertoElias Following the discussion we've had, here's a suggestion for the v2 of buttons, using a simpler API, making it easier to deprecate stuff over time.

This PR also deprecates old pagination/group classes because they break the BEM conventional naming:

The `.o-ft-buttons__pagination` wasn't following the BEM convention properly and will be removed in next major release. Use `..o-ft-buttons-pagination` instead.
The `.o-ft-buttons__group` wasn't following the BEM convention properly and will be removed in next major release. Use `.o-ft-buttons-group` instead.


Here's the generated CSS in non silent mode. There's a few lines of code that could be removed from the output by adding some filters in the Sass mixins but I preferred keeping the Sass very simple.

```css
.o-ft-buttons {
	box-sizing: border-box;
	display: inline-block;
	margin: 0;
	-moz-appearance: none;
	-webkit-appearance: none;
	text-decoration: none;
	text-align: center;
	border-width: 1px;
	border-style: solid;
	border-radius: 5px;
	outline: 0;
	user-select: none;
	cursor: pointer;
	transition: background-color 0.3s;
	padding: 0 14px 0 14px;
	font-size: 14px;
	line-height: 22px;
	background-color: transparent;
	color: #2e6e9e;
	border-color: #2e6e9e;
}

.o-ft-buttons[aria-selected=true] {
	background-color: #74736c;
	color: #fff;
	border-color: #74736c;
}

.o-ft-buttons[aria-selected=true]:focus {
	box-shadow: inset 0 0 3px 1px #fff;
}

.o-ft-buttons:focus {
	box-shadow: inset 0 0 3px 1px #2e6e9e;
}

.o-hoverable-on .o-ft-buttons:not([disabled]):not([aria-selected=true]):not([aria-pressed=true]):hover {
	transition: background-color 0.3s ease 0;
	background-color: rgba(167,165,155,0.15);
}

.o-ft-buttons[disabled] {
	cursor: default;
	opacity: 0.4;
}

.o-ft-buttons--medium {
	padding: 0 14px 0 14px;
	font-size: 14px;
	line-height: 22px;
}

.o-ft-buttons--big {
	padding: 0 18px 0 18px;
	font-size: 16px;
	line-height: 34px;
}

.o-ft-buttons--standard {
	background-color: transparent;
	color: #2e6e9e;
	border-color: #2e6e9e;
}

.o-ft-buttons--standard[aria-selected=true] {
	background-color: #74736c;
	color: #fff;
	border-color: #74736c;
}

.o-ft-buttons--standard[aria-selected=true]:focus {
	box-shadow: inset 0 0 3px 1px #fff;
}

.o-ft-buttons--standard:focus {
	box-shadow: inset 0 0 3px 1px #2e6e9e;
}

.o-hoverable-on .o-ft-buttons--standard:not([disabled]):not([aria-selected=true]):not([aria-pressed=true]):hover {
	transition: background-color 0.3s ease 0;
	background-color: rgba(167,165,155,0.15);
}

.o-ft-buttons--standout {
	background-color: #2e6e9e;
	color: #fff;
	border-color: #2e6e9e;
}

.o-ft-buttons--standout:focus {
	box-shadow: inset 0 0 3px 1px #2e6e9e;
}

.o-hoverable-on .o-ft-buttons--standout:not([disabled]):not([aria-selected=true]):not([aria-pressed=true]):hover {
	transition: background-color 0.3s ease 0;
	background-color: rgba(39,94,134,0.15);
}

.o-ft-buttons--inverse {
	color: #fff;
	border-color: #fff;
}

.o-ft-buttons--inverse[aria-selected=true] {
	background-color: #fff;
	color: #505050;
	border-color: #fff;
}

.o-ft-buttons--inverse[aria-selected=true]:focus {
	box-shadow: inset 0 0 3px 1px #505050;
}

.o-ft-buttons--inverse:focus {
	box-shadow: inset 0 0 3px 1px #fff;
}

.o-hoverable-on .o-ft-buttons--inverse:not([disabled]):not([aria-selected=true]):not([aria-pressed=true]):hover {
	transition: background-color 0.3s ease 0;
	background-color: rgba(255,255,255,0.15);
}

.o-ft-buttons--uncolored {
	background-color: transparent;
	color: #74736c;
	border-color: #74736c;
}

.o-ft-buttons--uncolored:focus {
	box-shadow: inset 0 0 3px 1px #74736c;
}

.o-hoverable-on .o-ft-buttons--uncolored:not([disabled]):not([aria-selected=true]):not([aria-pressed=true]):hover {
	transition: background-color 0.3s ease 0;
	background-color: rgba(0,0,0,0.15);
}

.o-ft-buttons__pagination .o-ft-buttons,.o-ft-buttons-pagination .o-ft-buttons {
	padding-left: 5px;
	padding-right: 5px;
	min-width: 24px;
}

.o-ft-buttons__pagination .o-ft-buttons.o-ft-buttons--big,.o-ft-buttons-pagination .o-ft-buttons.o-ft-buttons--big {
	padding-left: 9px;
	padding-right: 9px;
	min-width: 36px;
}

.o-ft-buttons__group>.o-ft-buttons,.o-ft-buttons-group>.o-ft-buttons {
	border-radius: 0;
}

.o-ft-buttons__group>.o-ft-buttons+.o-ft-buttons,.o-ft-buttons-group>.o-ft-buttons+.o-ft-buttons {
	border-left-width: 0;
}

.o-ft-buttons__group :first-child,.o-ft-buttons-group :first-child {
	border-bottom-left-radius: 5px;
	border-top-left-radius: 5px;
	border-left-width: 1px;
}

.o-ft-buttons__group :last-child,.o-ft-buttons-group :last-child {
	border-bottom-right-radius: 5px;
	border-top-right-radius: 5px;
}
```
This is too indented