Support for registering less functions

[nevir]

It'd be handy to give us a place to declare additional functions to be made available to less (or maybe just exposing the instance of less used). Probably means pulling less ^2.0.0, though. Getting custom functions into 1.x is a bit messy.

[forivall]

👍

[winstliu]

Just to confirm: does Less versions 2 and up (say, 2.7.1) have custom functions support?