wip: Custom Mapping Support

[hYdos]

Enables the ability to use custom mappings within VanillaGradle.

TODO:

• Remap Dependencies and somehow figure out what mappings they use
• Figure out how the user will set what mappings to use
• Figure out how jar building will work. (Will it stay named? Will their code be remapped into obf?)

[zml2008]

Superseded by #47, thanks for getting this started though