Add texmf path setting and search the corresponding path for bib files

Currently, LaTeXTools doesn't find the "obvious" place to put one's bib files - the bibtex/bib subdirectory of the texmf tree. As a workaround, this patch adds a "texmf" setting to the LaTeXTools preferences, which can be used to specify the texmf path. LaTeXTools will then also search the bibtex/bib subdirectory of that path for bib files for the /cite panel etc.

Of course, LaTeXTools should ideally search all the directories for bib files that LaTeX itself does when building. That would require querying kpsewhich for these paths (or something equivalent to that). This patch doesn't do that - I'm sharing it as a stopgap for people who need this functionality immediately ;)