Added git-repo support for packages not on npm

[JoernBerkefeld]

for issue #186 features

• added new dependency to my library git-version-tag (full support for GitHub, Bitbucket, GitLab)
• added new constructor option "remoteUrl" to specify the git repo-url
• auto-switching the printed command from install to "update" of install command for git-based packages
• Docs: made options more easily readable (using header tags alone doesn't really let you see where things belong to with the current GitHub styles)

[papb]

Hello, what was the problem with is-installed-globally?

[JoernBerkefeld]

We tried it on 2 different Windows 10 machines and it didn't work. The docs for the underlying package are pointing to the npm command which returns the right global dir for my team's computers. However, internally that package was using environment variables instead that for whatever reason pointed elsewhere.

[sindresorhus]

I appreciate the pull request, but this is not something I want to maintain. I recommend publishing a fork with your changes instead.