Make the package a Flask extention so it can be used with init_app

[sergioisidoro]

When making modular flask applications, it's common not to immediately provide the app in the constructor, but to pass it later on with the method init_app(app) as we can see in the documentation -- https://flask.palletsprojects.com/en/1.1.x/extensions/

With a bit of changes on the constructor this can become a "proper" flask extension
https://flask.palletsprojects.com/en/1.1.x/extensiondev/

The example of the SQLite extension provides an idea on how this could look like:

    def __init__(self, app=None):
        self.app = app
        if app is not None:
            self.init_app(app)

    def init_app(self, app):
        app.config.setdefault('SQLITE3_DATABASE', ':memory:')
        app.teardown_appcontext(self.teardown)