Configuration file support and manpages

Released on January 14th, 2016.

• Issue 2: Added a manual page.
• Added support for a configuration file. The default is to load
  ~/.config/markdown2social.conf if it exists, and the path can be
  overridden via the --config_file flag.
• Added support to specify a set of "replacements" to apply to the post
  text after conversion. This is useful, for example, to automatically
  turn bare references to external systems into links.