Class-based directives

[gtxm]

Hey, it's my first contribution here, so let me say: awesome project, I love it, thank all of you for it!

I used hug for my project and I found a problem how to include a database connection (in my case sqlalchemy session) to the project. While searching for the solution I found some issues that also mention this problem: #376 and #458.

In the PR I included my solution to the problem - a class-based directive with a method for a cleanup when something goes wrong. In examples I included two use cases: sqlalchemy and mail sending. I will be happy to include more.

@hug.directive()
class SMTP(object):

    def __init__(self, *args, **kwargs):
        self.smtp = envelopes.SMTP(host='127.0.0.1')
        self.envelopes_to_send = list()

    def send_envelope(self, envelope):
        self.envelopes_to_send.append(envelope)

    def cleanup(self, exception=None):
        if exception:
            return
        for envelope in self.envelopes_to_send:
            self.smtp.send(envelope)


@hug.get('/hello')
def send_hello_email(smtp: SMTP):
    pass

One nice thing about solution is that annotation still preserves its basic functionality - a smtp variable is an object of SMTP class.

Any comments are welcome, thanks!

[coveralls]

Coverage increased (+0.001%) to 99.845% when pulling 7ebcb60 on gtxm:class_directives into bf311e7 on timothycrosley:develop.

[timothycrosley]

This looks fantastic, thank you!