[#21] Wrote test detailing expected behavior of plugin

- Missing Input Filter Param In Event Causes Plugin To Yield Null
- Invalid Type In Event Input Filter Param Causes Plugin To Yield Null
- Valid Input Filter In Event Is Returned By Plugin
[#21] Implemented plugin
[#21] Register plugin with controller plugin manager

- as an invokable, under the key 'getinputfilter'
[#21] Documented plugin in README
Merge branch 'feature/21'

Close #21