Add support for bootstrap4 custom breakpoints

[jhotadhari]

Hi,
its just a proposal.
I don't need that feature actually. no rush.

Added a function to extract all bs4 breakpoints from from :root style properties.

Why? Usecase

For example a project, that overwrites the $grid-breakpoints variable:

$grid-breakpoints: (
  xs: 0,
  xssm: 411px,
  sm: 592px,
  md: 782px,
  lg: 992px,
  xl: 1200px
);

Results to this :root style properties:

The custom breakpoint icon will use the default text color:

Disadvantages

Couldn't find a direct way to extract the breakpoint properties. Therefor its done with a loop, which takes time.
For me, it's around 5-8ms per pageload.

May be this feature might be better to opt out on default.

[necodeus]

I love the idea. You could also add a way to customize breakpoints via extension settings.