Allow variables to be set within conditional sections

[csytsma]

It appears that variables can only be set as a stand-alone line. For example:
@color: red;

It would be great if the variable assignment could happen within a conditional, so the value could be set based upon the element being rendered. For example, with [name] being set in the layer data:

[name='trek'] { @color: red; }
[name='bike'] { @color: blue; }
[name='canoe'] { @color: green; }
line-color: @color;

[Komzpa]

@csytsma this should be reported towards https://github.com/mapbox/carto.

[csytsma]

Created mapbox/carto#461