make installation script configurable for forks

[ayandyl]

This branch replaces the hardcoded robbyrussell repo owner with a configurable variable in the install.sh script, so that fork maintainers (or private forks) can replace it with their namespace, and install the forked version of oh-my-zsh pointing at their fork.

This preserves original behavior by defaulting the said variable to robbyrussell.

[mcornella]

Will be fixed by #3648.

[ayandyl]

noted thanks @mcornella

[mcornella]

#5169 now pulls in the ability to install from forked by setting these variables:

The installer respects the following environment variables, that can be set up when calling it with VAR=value sh install.sh.

• REPO: value in the form of owner/project. Useful for forks in GitHub (defaults to `robbyrussell/oh-my-zsh).
• BRANCH: default branch to check out once the clone has finished (defaults to master).
• REMOTE: full remote URL (supersedes REPO if set). You could for example choose an ssh remote URL, like git@github.com:user/oh-my-zsh, or another fork from GitLab or Bitbucket.

If you want to help speed up the testing and merge (which should happen early next week), you can wget or curl it from the URL https://raw.githubusercontent.com/mcornella/oh-my-zsh/refactor-installer/tools/install.sh. Read the comments at the top of the script to know how to call it.

Thanks!