Add `pip home` command that opens a package's homepage in the default browser

Very useful for quickly getting to package's project homepages. Like homebrew's `brew home`.

Example:

    > pip home pip
    Opening pip's homepage in browser
      https://pip.pypa.io/

Like `pip show`, it can take multiple arguments, quietly errors if the argument(s) represent package(s) not found on the system, and errors with a message if no arguments are given.
You shouldn't need to import and instsantiate this manually, just add a `monkeypatch` argument to test functiont that need it.

Doing it this way means that stuff isn't rolled back correctly.