Mostly cosmetic changes and small fixes.

- Added [Arguments] section to plugins metadata.
- Moved metadata to golismero/__init__.py so it's accessible all over the
  program.
- Lots of cosmetic changes to golismero.py and the plugin metadata.
- Fixed setup.py and moved optional requirements to
  optional-requirements.txt.
- Added required version of BeautifulSoup to requirements.txt (see #110).