Added ShellSession language

This uses the pygments.rb Bash Session lexer to colorize shell sessions:
https://github.com/tmm1/pygments.rb/blob/master/vendor/pygments-main/pygments/lexers/shell.py#L110

I chose the `.sh-session` extension because that is what pygments specifies.  

I did not include an updated `samples.json` because when I ran `bundle exec rake samples` it changed a lot more than just my stuff.