Allow user to exclude paths in the repo, and group lines changed by file type

Added two new arguments:

1) **exclude** Allow the user to exclude certain paths in the repo. In the case of a Rails 4 project, excluding vendors or config/locales might be desirable to get a more realistic idea of authors' contributions

2) **bytype** Allow the output of line counts by file type (ruby, erb, yml, js). In the case of git-fame-rb, 7rans wrote the rdocs and Linus Oleander wrote all the ruby. This is especially helpful on larger teams/projects. [ unknown files are files without an extension. If you can think of a better way to represent these, have at it! ]

I also wrote a few tests (though rspec isn't my strongest suit) and modified the readme. I believe my addition about . mailmap is warranted since exactly where those author names come from is a mystery (unless one digs into the gem, and understands git shortlog).

Example of line count by file type output:
![screen shot 2013-11-13 at 10 55 53 pm](https://f.cloud.github.com/assets/515046/1538304/b1c3e09c-4ce0-11e3-9023-f994e5384590.png)