Add optional `$color` argument to `button-variant()` and
`button-outline-variant()` for additional flexibility.

By the time v5 stable will be out, 8 will be EOL (December 2019).

This way we match the Travis CI behavior, and it's inherited by all steps.

* Scope .form-check-input layout to the .form-check parent

* Remove margin-bottom from list-group-items in case they are label elements

* document .form-check in input groups and list groups, and cleanup the no-label examples in the main .form-check docs

This section is meaningless and doesn't serve any purpose.

Without this, coveralls didn't receive the branch info thus leading to failures when we switched branches.

* Remove outline from select box in FF

* Use $form-select-color

Stack in cards in one column when < md

Now there's only one command needed to run the docs: `npm run docs-serve`.

Also, simplify the npm scripts.