Find Markdown.tmlanguage even if it's not at the usual location.

Figure out at runtime where the Markdown.tmLanguage file is. This is useful when the MarkdownEditing package is installed, which disables the built-in Markdown package and replaces the tmLanguage file with its own. Since the file then is found at a different path, Sublime Text fails to set the syntax. Now it finds the file regardless of its location. The original location is still used as a fallback.