add initial cgit plugin

This pull request contains a simple plugin for the detection of cgit.

```
$ ./whatweb git.kernel.org
http://git.kernel.org [301] HTTPServer[nginx], IP[149.20.4.72], RedirectLocation[https://git.kernel.org/cgit/], Title[301 Moved Permanently], nginx
https://git.kernel.org/cgit/ [200] HTTPServer[nginx], IP[199.204.44.194], Title[Kernel.org git repositories], cgit[0.10.1], nginx
```