duplicate functionality in merge! and hcat!

[cjprybol]

AFAICT merge! and hcat! do the same thing. Should we export hcat! and deprecate merge!?

[ararslan]

If anything I would do it the other way around: keep merge!, since that's the familiar name for this operation, and deprecate hcat!, since that's more reminiscent of Julia matrix terminology.

[cjprybol]

Sounds good to me! It's also easier to change the internal function names than it is to change all the code out in the wild that uses merge!. When I open the PR for this I'll add something to the docs to help users choose between hcat, merge!, and full outer join which all allow users to merge tables but do so in different ways. I just found out about merge! while reviewing tests for the #30 refactor.