Automatic version download

[Yurii Rashkovskii]

It would have been really cool if exenv was able to automatically download/build (using git?) specified but missing elixir versions, provided version is specified in one of these formats:

• vx.y.z (like v0.9.1)
• vx.y.z-n-sha (like v0.9.1-107-g2b8854c, aka git describe format)
• sha (like g2b8854c)

[Yurii Rashkovskii]

oh I didn't realize there was https://github.com/mururu/elixir-build

[Yuki Ito]

@yrashk Yes, but I think it may be better to merge them now.