API for future Minecraft Development plugin integration

[RedNesto]

I would like in the future (after some long-due refactorings on our side) add support for VanillaGradle to MCDev.

To do so I would need a stable way to access some information, namely:

• the target Minecraft version
• the mappings file, bonus point if the plugin can output a SRG file (that would significantly simplify things for us, might be related to Remapping inputs and outputs #34)
• the access wideners files (not sure if we really need it yet)
• the name of the "decompile" task to run, if needed (can be null if the user configured the plugin to always decompile for example)

Note that this is not a definitive list, I'll have a better idea of what I need exactly when I start working on it.

To import data we have access to the Gradle Project, meaning we can use anything stored in plugins/extensions/configurations. If it can help, see how we get these information from ForgeGradle