Support fetching binaries

[ZoomTen]

At the moment, you need the RGBDS build dependencies to be able to install versions, since it has to compile them on the machine it's running on.

[ZoomTen]

As of writing:
Win32 binaries available since 0.1.0
Win64 binaries available since 0.2.5
Mac binaries available since 0.6.0
Linux binaries available since 0.6.1

All of the above assumes the GitHub releases page

[ZoomTen]

An attempt at compiling every version that rgbenv currently covers (really, the tagged releases): https://github.com/zoomten/a-lot-of-rgbds

At time of writing, currently compiles statically for x86_64 Linux and x86 Windows with a few runtime problems. With gbdev.io's blessing and probably a better way to go about this, this might be a good source for binary download.