fix missing markdown tmlanguage error

previously, the plugin would arbitrarily set a syntax to a file that did
not necessarily exist, which caused an error like

> Error loading syntax file "Packages/Markdown/Markdown.tmLanguage": Unable to open Packages/Markdown/Markdown.tmLanguage

instead, the plugin now searches for a Markdown.tmLanguage file using `sublime.find_resources(glob)` and logs whether or not it succeeds in finding one