Allow installing from GitHub branches

[ngokevin]

Currently, attempting to install a GitHub branch will break because multidep uses the version name for the directory name. GitHub install looks like user/repo which has a slash, which will break.

I propose specifying an array. The first value is the install target, the second is the directory suffix.

“versions”: {
  // Will be installed at .multidep/mypackage-thatbranch
  “mypackage”: [[“myuser/mypackage#thisbranch”, “thatbranch"]]
}

My use case is to use multidep for installing versioned documentation. The documentation is versioned on NPM, but would like to retroactively apply documentation updates using GitHub branches.

I'm going to work on this now. Let me know if you have suggestions!