rowkey source extension for extrachHbaseCells command

Hi,
I've created a new pull request with rowkey source extension for extrachHbaseCells command. I've tried to do changes so little as possible but i've come to the conclusion that is necessary to separate the abstraction of extraction. On the first shot if that was all in one Mapping class that was a mess spaghetti code. So I am sorry for the refactoring and I hope you will like it.