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.
Store the subtitles from edX to the destination directory rather than the working directory to fix a bug in #53 and #43 .
Skip subtitles if the video files are not downloaded to avoid the confusing. (#53)
Simplify the subtitle options by removing `--youtub-subs` and `--edx-subs`.
Merge pull request #60 from shk3/subtitle_fix

Recheck the subtitle fallback part and correct the bugs in #53
Merge branch 'master' into argparser

Conflicts:
	edx-dl.py
Merge branch 'master' of https://github.com/shk3/edx-downloader
Small cosmetic changes and pep-8 love
Rewrote for clarity parser arguments for clarity
Refactoring + updated options, and simplified subtitle management

- removed that ugly 'global' stuff, all options are self-contained in the
  args object now
- changed output directory from -d to -o to be more consistent with common
  command line settings where -d use to be for debug and -o for output
- reduced options to only one short name and one long name.
- removed the user-agent option since it's not part of the current
  interactive mode, and I don't think the end user must care about this
- refactored and simplified the subtitle handling to reduce complexity
Added direct course download option
Merge branch 'argparser'

Conflicts:
	edx-dl.py