Clamp rgb(R, G, B, A) to rgb(R, G, B)

This is what upstream lessc does.
Change color 'BLUE' to 'blue'

Upstream lessc only knows lowercase color names, we should stick to
that.
Add argb() function

Used by bootstrap3. Quote from the lesscss.org documentation:

Creates a hex representation of a color in #AARRGGBB format (NOT
Android development.