Provide CLI options for extract_swift_flags.py

3 options:
  - location of the swift repo
  - location of the manuals repo
  - verbosity

The defaults should be sane to use with the soon-to-be-updated
autohelp-wrapper.

Change-Id: I2395aa5c6bfdb30990ec7f930a9277cdef6d3777
Merge "Provide CLI options for extract_swift_flags.py"