Shim for easy upgrade from v3 to v4

[jahlomp]

This will make it very easy to still use bootstrap 3 while progressively migrating to v4.
The Font Awesome theme has a nice example of this at https://fontawesome.com/how-to-use/upgrading-from-4

[Johann-S]

Not sure if we will have time to implement that 🤔

[Herst]

Not sure whether improving Bootlint in this direction wouldn't be a more reasonable (and less error-prone) approach. I am personally no friend of such client-side shims.

[mdo]

Sorry, but this just isn't feasible or productive—for us or folks using Bootstrap. v3 doesn't go away, and v4 is such a sweeping overhaul. Providing a shim would be bloated, slow, difficult to produce, etc.

[jahlomp]

OK. Thanks.