Move Processing 3 build commands into named variants

This pull request combines the two separate build systems into one, relegating the Processing 3 build commands into variants (accessible via the ST command palette).

The restores Processing 2 as the default build system so things work out of the box using ST's Automatic build setting. Fixes issue #57, references #51.

@mckennapsean Can you live with this until Processing 3 is launched?