Add support for custom grammars

[BluSpring]

The Browser Transcription system supports adding custom grammar via the JSGF file format, however it may be slightly inaccessible for many users.

Additionally, we cannot guarantee that servers will be able to safely provide grammar definitions without them trying to potentially overload user memory (lots of string data stored within both Minecraft and the browser), which could lead to disastrous consequences.

However, we are currently trying to implement custom grammar support no matter what, but it may only be available on the client-side. We're still not sure.

[BluSpring]

Some experimental support for this has been implemented in the feature/custom_grammar branch, which doesn't seem to work as I originally expected.. I do not think I actually understand the grammar system correctly, and may need a bit of help with it.