Merge upgrades

This pull request addresses issue #65

Added support for commit messages (-m) and fast-forward only enforcement (--ff-only) to git merge.
1) no, I think it could be confusing
2) yes, makes sense to me, but again a little confusing.
3) You could add three constants in the MergeCommand class like in the [StatusFile](https://github.com/matteosister/GitElephant/blob/develop/src/GitElephant/Status/StatusFile.php#L29-L37) class for example 

What about adding two methods? **merge()** and **mergeFF()** Maybe a little overkill?