add shellchecker

Hi,
It would be nice, (at least for me) if I could use code sniffer to check on my bash scripts.
I use this http://www.shellcheck.net/  tool but I don't have it available on my IDE, SO if we could merge this in CodeSniffer that would be the easier solution for me.

I just made the enough to get it working, so no real Tokenizer as this is not needed since this tool already takes care of that. 

I would like to know what do you think about it. Should CodeSniffer sniff bash scripts also?
I'm open to other solutions or to improve this PR. (I'm also already trying to create a plugin for my IDE).

Thanks