Symlinks the 'github' CLI binary that enables opening GitHub for Mac
from the command line. As per the defaults for the application, the
internal `github_cli` binary is symlinked simply as `github`.
Note this intetionally does *not* also symlink the bundled versions.
of `git` itself, as anyone using brewcask is going to want to be able
to manage those normally via homebrew.`
`md5`, `sha1`, `sha256` all take a hexdigest string, e.g:
sha1 'f645e9da45a621415a07a7492c45923b1a1fd4d4'
`no_checksum` takes no argument, and indicates there is no checksum
for this cask. This is *not recommended*, and should only be used for
casks that have no versioned downloads.
`brew cask install` will complain if there is no sum provided (unless
`no_checksum` has been invoked), or if the sums do not match. It will
provide the computed checksum so the cask can be easily amended.
Adapted from @passcod's work in 82cc199ae6
Vítor Galvão