Add support for Less Maps

[aviemet]

Hi, I created an issue for this, but then just decided to implement it. This adds support for Less Maps, and also will resolve variables which are not at the beginning of the string. So, something like darken(@blue, 20%) or 1px solid @blue would resolve to the variable value if found in the document or provided dictionary.
Thank you,

[coveralls]

Coverage remained the same at 100.0% when pulling d5fc597 on aviemet:master into 7844ed3 on michaeltaranto:master.