feat(dropdown): add alignment option

I have added "alignElements" function in position.js for aligning horizontally a target element to a host element, in the same style than "positionElements".

I make use of it in dropdown.js for aligning the dropdown menu with the dropdown toggle element.
`<ul class="dropdown-menu" dropdown-align="right">`
Possible dropdownAlign values are:
- left
- right
- left-auto (like left but with no overflowing on the right)
- right-auto (like right but with no overflowing on the left)
- center

The idea is -if this pull request gets accepted- to apply the same to type-ahead.

I have not added tests yet, please tell me if they are required.