Better PyInstaller integration, improved `release` command

There are cases where you simply need to supply extra arguments to fbs's invocation of PyInstaller. There is now a new setting, extra_pyinstaller_args, that lets you supply those parameters. For example, in base.json:

{
    ...,
    "extra_pyinstaller_args": [
        "--additional-hooks-dir", "some_hooks",
        "--additional-hooks-dir", "some_more_hooks",
    ]
}

Fixes #157 and #167.

The release command was also improved. You can now use any of the following command lines:

fbs release
fbs release 1.2.3
fbs release current

Fixes #211.

Thank you to @whattheserver and @luigifreitas for their PRs.