All over jekyll, there's code to prepend / append a slash or to remove a leading / trailing slash (mostly from paths, of course).

I'm thinking about adding some kind of `PathHelper` class (or module?) for these methods. This could also handle the detection of `destination` within `source` (#1130) and possibly others (#1297?, #1184?). Also, my PR for default layouts #969 also works with this kind of stuff.

Good / bad idea? Class / Module / extension to `String`?