Add support to install go from source

[urso]

• Building go is optional. If binaries are installed only, no source code is ever checked out or downloaded
• Source code is checked out as bare repository into ~/.gvm/cache/go
• To force building from source pass -b flag to use or install command (version is only build if not already installed)
• Installer tries to get binary version first. Only if this fails we fall back to building from source (local source checkout can act as cache)
• Version tip is always build from source
• When using tip, the cache is only updated once per day. tip is rebuild if new commits are found in the repository
• Introduce gvm init command. init clones the git repository or forces a git fetch.
• Expose some more flags like target os, arch, gvm home directory, ...

Building from source requires an active go environment (use gvm to install a binary) and git for checkout. On Windows cmd /C make.bat is used for building. Other systems will run bash make.bash.

[ruflin]

I assume title should say Go from. source?

[urso]

I assume title should say Go from. source?

Yeah. After typing go,git,gvm for hours in terminal it's become too easy get use the wrong one :)

[andrewkroh]

This is awesome!

[andrewkroh]

I tried out gvm use tip on windows. 👍