added mixin hinting functionality

[dularion]

In this pull request I added functionality that gives the user hints for mixins as well as variables. All the old variable-hinting is still intact, nothing was changed there except for a little colored label in front of the hint to indicate, that the hint represents a variable, not a mixin.

in order to get variable hints, just type $ like before. in order to get mixin hints, type @. Leave out the include though, as it will start filtering the results. It will be added for you anyways. All the variables in the mixins are preserved, so that the user can see which variables he/she has to pass to the mixin in order for it to work properly.

In order to get this to work, I listen for both the $ and @ triggers, and I use 2 different regex to look for the specific hints. I tried to comment everything as much as possible, so that everything looks and feels the way the original programmer intended.

[tfevens]

This would close issue #3.