Auto extend with Sanic Extensions

[ahopkins]

This PR makes Sanic Extensions automatically setup when it is in the environment. This will make it much easier for anyone that wants to add this funcitonality:

pip install sanic[ext]

If a developer wishes to opt out of this behavior:

app.config.AUTO_EXTEND = False

The goal is to replace this pattern:

from sanic import Sanic
from sanic_ext import Extend

app = Sanic("MyHelloWorldApp")
Extend(app)

With this:

from sanic import Sanic

app = Sanic("MyHelloWorldApp")

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Any configuration that would have occurred on the Extend object can now be done:

app.extend(...)

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TODO

• Support early usage of app.ext before it is setup
• Fail gracefully when manually extended

[codeclimate]

Code Climate has analyzed commit 8cbdbe1 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 96.8% (86% is the threshold).

This pull request will bring the total coverage in the repository to 86.9% (0.3% change).

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