Selectivity: Wire-up with CssSourceMap

**Target content-types:** LESS and SCSS.

**Description:** With compile on save, .css and .map
files generation options are enabled, opening
the file and pressing save will generate parsed
source maps. Thereafter, hovering over selector
will calculate selectivity from corresponding
generated selector.

**Remarks:** We may generate the CSS and map files
at temporary location on opening LESS/SCSS
documents regardless of settings; to collect
the compiled version of each Selector. This will
help us adding rich, persistent features.