Implement 'github' repo support

[evanplaice]

Implement support for installing the direct source from a repo in src/util/github.js.

There are comments in that file that pinpoint some API endpoints that need to be hit for the relevant issue.

Like NPM, GitHub repo releases will also have a link to a .tgz files and their corresponding shasums.

To install this:

• figure out which damn version the user wants to install (ie latest or provided version)
• fetch the latest version or look up the versions and ensure the provided version is valid
• pull the tag details (ie .tgz uri + shasum)
• download, verify and unpack the package¹
• update the config

¹ The untar includes an override that strips the parent directory from every directory specifier. This may need a flag to enable/disable if GitHub sources aren't wrapped with an outer directory before being tar'd.