Detect and import files with a .rumor extension

[exodrifter]

Unity has an experimental feature called Scripted Importers that allows a program to load file extensions not natively supported.

This feature will allow a user to save rumor files in a custom extension (like .rumor) instead of .txt. This will also be useful for #50, #51, and #52, since syntax highlighting is often automatically enabled based on the file extension.

For now, this feature should just import any .rumor files into something similar to a TextAsset. In the future, it may be possible to do more complex things like precompile the script.

[exodrifter]

Completed in b404dd7.