Added ability to merge/rename authors

Added ability to merge/rename authors

There's a new config field, 'merge_authors', which is a dictionary of
source name to target name. Whenever an author name matches a source
name it will be treated as if it was the target name instead.

Use this if authors have committed under multiple names, to squash their
statistics down to a single author. You can also use it to rename an
author for the purposes of the output.

Additionally, the -c option has been extended so for a dictionary option
you specify -c field=key,value. The key,value pair is then ADDED to the
dictionary.

Putting it all together in an example:

  ./gitstats -c merge_authors=bob,Bob\ Jones    \
      -c merge_authors=bob2,Bob\ Jones          \
      -c merge_authors=erica,Erica\ Smith ....

Signed-off-by: Heikki Hokkanen <hoxu@users.sf.net>