Noise gate plugin (Noise reduction)

- Adds a microphone noise gate plugin with a configuration window.
- Adds OBSGetCur*VolumeStats() API calls to get real-time RMS, max and peak volume information from a plugin.
- Adds support to display two volume bars on the VolumeMeter control but as nothing uses this yet so there is no visual change.

Fixes issues #51 and #127.