Release Api module

Merge these first, they add a couple more bits to the Api module.

- [] #29
- [] #30 

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.