Check whether user-supplied path is a rust doc directory

From discussion at issue #5.

I've barely ever used Ruby, so the code might not be fully idiomatic; I do prefer the "if !..." to "unless ...", though, since there is an else branch.

So this really improves two things IMO: one, it refuses to copy any directory the user specifies (previously it would attempt to copy even /), and two, if the *root* of a rust source directory is specified, it auto-corrects to the doc directory.