[v1] (Possible) checks to be added

[Herst]

Feel free to discuss and vote with your thumbs.

[Herst]

"should add a warning when Popper.js isn't imported, except if they use the bundle file" (suggestion from Johann-S in https://github.com/twbs/bootlint/pull/419#pullrequestreview-156319666)

[Herst]

Check for Bootstrap 3 classes: For a quick-and-dirty attempt see https://github.com/Herst/bootlint-ng/blob/v4-old-attempt/src/bootlint.js#L316

[Herst]

Input-group minutiae like .custom-select required to make <select>s to work, see #423 (replacement for E006)

[Herst]

Multiple inputs in an input-group only partly working, see #423 and https://getbootstrap.com/docs/4.1/components/input-group/#multiple-inputs, a case for a warning? (replacement in E010)

[Herst]

Tests for .card-group/.card-deck/.card-columns and making sure that .card-title/.card-subtitle are added to a <h*> and .card-link to <a> elements (#425).

[Herst]

#189

[Herst]

#356