Visibility mixin

[cortopy]

The docs mention an invisible @mixin which is not present in the source scss files.

This adds a visibility mixins file, and references it in the utilities file.

[houndci-bot]

!important should not be used

[cortopy]

This !important was in the original file

[tomlutzenberger]

I'm not sure if it's really necessary to add an extra mixin with only 1 rule.
It would increase the code to maintain and add one more file.
IMHO this is an overkill, the .invisible class would be enough.

[cortopy]

@tomlutzenberger this mixin is mentioned in the docs: http:https://v4-alpha.getbootstrap.com/components/utilities/#invisible-content

The way I'm using it is by creating my own class with the three mixins for an accessible invisible class. I thought the docs made sense, as you may or may not want to hide for screen readers too. Without the @invisible mixin my class doesn't work on absolute positioned elements

.hidden
  @include invisible
  @include sr-only
  @include sr-only-focusable

This may not be a good idea for some reason but then the docs need amending

[mdo]

Thanks!