allow functions to be passed by name into the schema

[richbai90]

Resolves #24

[eiof]

Your open issue lead us to exploring the possibility of plugin support, so we won't be accepting this PR for now. If you need this functionality for now, you can use your fork. I won't have access to a computer for a week starting tomorrow, so I won't be able to look at plugin support for awhile.

We'd be interested in a plugin for this functionality once we have a better of what that'd look like. Thanks for helping us improve this library!