Add support for using McTester from a Sponge plugin

[Aaron1011]

Support for running McTester out of SpongeForge is mostly there - in fact, this repository tests itself via ExceptionTest. A few changes need to be made before it's fully complete:

• Properly clean out the 'mods' directory when moving from a non-Sponge runner (which requires us to download the latest Sponge jar) to a Sponge runner (where we use the Sponge sources that are already on the classpath). Right now, we end up with a leftover spongeforge jar when running under Sponge.

• Document and parameterize the sponge downloader. We currently always download the latest SpongeForge 'stable' build for 1.12.2, but plugins may want to customize this (e.g. test against a fixed build or the recommended build).