Provide tab completion for Bootstrap Sass variables

[cpsievert]

Using ?utils::rc.options, and maybe borrow some ideas from https://github.com/jimhester/completeme

[cpsievert]

Also, if completion isn't possible, we should throw an error/warning if we receive a variable name that we don't recognize.

[cpsievert]

Maybe the best way to handle this is to just provide a bs4_theme() and bs3_theme() which explicitly list all the variables in their arguments (and we'd have the update script to automatically parse Sass vars and transpile into functions)

[schloerke]

(and have the function start with ...)