Method to add multiple includes: build.includes(...)

[dtolnay]

Closes #352.

This adds an includes method to call include(...) for each dir in an iterator, matching the files method from #205 which calls file(...) for each source file in an iterator.

My immediate use case for this involves a bunch of dirs that may or may not be selected (Option<PathBuf>) so that's what I put in the documentation, but any other iterator would work.

[alexcrichton]

Looks good to me, thanks!