Make DirectoryInput part of the pubic API

As clients of the gem will need to call it, to then call Api::Flow, they shouldn't
have to call something in the Parser directory.

Moving the DirectoryInput module itself was a bit of a faff, so for now i've effectively
aliased it, so it's officially part of the public API, and the smartdown internals can
be rearranged later
Merge pull request #30 from alphagov/hide-directory-input-behind-api

Make DirectoryInput part of the pubic API