Add --print-config option.

Useful for debugging
Change --log_file -> --log-file
Tweak a help message
Replace Popen() with simpler call()
Replace call() with check_call()

Seems safer, Terminal Velocity will crash with a CalledProcessError if
the editor exits with non-zero exit status.
Hide traceback on ctrl-c

Fixes #24
Add --extensions option

Fixes #18