[feat] Add uncenter mixin

This PR is a suggestion for something that happened over and over on a project I'm working on.

The idea is simple, since I'm using Mobile-first approach to create all my styles, I came across the situation of using the "center" mixin a lot to center elements for small displays. When increasing the breakpoints values, I came with situations of having to reset over and over the "center" mixin declaration values. In that sense, I created the "uncenter" mixin, that basically sets the values to default. I thought that it could be useful for other people so I'm sharing it here. It proved to be useful in this project so because didn't have to write over and over the same code.

If this PR happens to be merged, the next would be update all the docs to mention this mixin.







Hey, this line is responsible for resetting to default the `max-width` value that the `center` mixin apply to the element. I'll try to illustrate with a real world example from a situation that happened to me:

I have the following code for social network on a footer element

```jade
ul.social-media
  li: a(href="#", target="_blank")
    +social-media__icon("#facebook")
  li: a(href="#", target="_blank")
    +social-media__icon("#linkedin")
  li: a(href="#", target="_blank")
    +social-media__icon("#instagram")
  li: a(href="#", target="_blank")
    +social-media__icon("#twitter")
```
and some style

```sass
.social-media
  center(223px)
  inline-list: 25px
  a, svg
    display: block
  svg
    fill: #000
  li
    border-radius: 100%
    border: 1px solid #000
    padding: 5px
```

This results in a centered block with  `max-width: 223px` because of the `center()` mixin. This is basically the style for my mobile devices, and ideally, I should use increasing breakpoints styles to deal with bigger devices, where in a certain breakpoint I eventually don't need that "center" functionality anymore because the layout is different or I just need a different behaviour, so I need to reset all the styles that the `center()` mixin applied to center the element. That's why `max-width: none` is used, otherwise the `ul.social-media` would still have `max-width: 223px` in that case which would not fit my desired behaviour.

I chose Jeet in my latest two projects and ran in this issue of resetting the `center()` in some situations, so I created a mixin to save my time which now I use as part of my custom mixins for each project, but I'd be really happy to see that as a built-in functionality.

If you have ideas on how to improve this, I'd be happy to hear