Add file-ext regex rules, exception handlers.

Add:
    bitshift/crawler/indexer.py
        -add two `try: except: pass` blocks, one to _decode() and another to
        GitIndexer.run(); bad practice, but GitIndexer has numerous unreliable
        moving parts that can throw too many unforseeable exceptions. Only
        current viable option.
        -add file-extension regex ignore rules (for text, markdown, etc. files)
        to _get_tracked_files().