add a function to return the number of items in a list

[lukeapage]

e.g.

@colors: red white green;
@colors-len: length(@colors); // <-- 3

see #1465 last comment

[radium-v]

(How) Would this work for comma-separated shorthand lists?

@transitions: border 1s linear, background-color 0.3s ease-in-out;

[lukeapage]

I suggest the same was as pick.. probably 2.. not sure what pick does in
that case though.

[seven-phases-max]

Added in 1.5.0