Change the address of youtube-dl.
Change the printed information wording.
Such questioning of subtitle sources seems unnecessary. If needed, how about putting it into options?
Change the method to fetch the name of video. It should fix #53.
Removed the commented code.
Use the os.path.join to generate the path.
Correct the way to add the extension of the subtitle path.
Make `.srt` part look better.
Avoid the repetition at the subtitle filename's generating.
Merge pull request #54 from shk3/subtitle_fix

Subtitle fix
Make the flag of downloading subtitles global.
Switch to argparse to do the parsing, which seems more powerful.
Allow control the subtitle downloading from arguments.
Allow to set video format by command.
PEP-8: Fix spaces before colon.
PEP-8: Fix more whitespace issues.
PEP-8: One last pass of whitespace fixes.
PEP-8: Remove trailing semicolon from end of statements.
Fix #58.
edx-dl: Fix appending to strings. Closes #58.

Python strings are immutable and they have no way of being "appended" or
having things appended to them.