Add variable support ($HOME) in .sublime-project && sublime-workspace

One of the most glaringly missing features (IMHO) is the lack of variable support in .sublime-project and .sublime-workspace files.

I sync my sublime projects directory across multiple systems.  This _only_ works on my personal systems where I have the exact same username and exact same folder structure, because the paths HAVE to be hardcoded due to the lack of support for variable expansion.