Add url_for method for simple routes, blueprints, HTTPMethodView

[subyraman]

Adds in a url_for method to generate URLs based on an endpoint's handler name, similar to Flask. Provides the same type of functionality as Flask, but will additionally fail URL building if a supplied parameter does not pass the request parameter pattern. From the docs I wrote:

Sanic provides a url_for method, to generate URLs based on the handler method name. This is useful if you want to avoid hardcoding url paths into your app; instead, you can just reference the handler name. For example:

@app.route('/')
async def index(request):
    # generate a URL for the endpoint `post_handler`
    url = app.url_for('post_handler', post_id=5)
    # the URL is `/posts/5`, redirect to it
    return redirect(url)


@app.route('/posts/<post_id>')
async def post_handler(request, post_id):
    return text('Post - {}'.format(post_id))

Other things to keep in mind when using url_for:

• Keyword arguments passed to url_for that are not request parameters will be included in the URL's query string. For example:

url = app.url_for('post_handler', post_id=5, arg_one='one', arg_two='two')
# /posts/5?arg_one=one&arg_two=two

• All valid parameters must be passed to url_for to build a URL. If a parameter is not supplied, or if a parameter does not match the specified type, a URLBuildError will be thrown.

Caveats: I did not try to make this work with static files or the CompositionView. The reason is I found no logical pattern for getting a view name for static handlers, since there is no unique "handler" surrounding a static file. Same for CompositionView handlers.

One possible idea would be to force the naming of a class based view, like Flask does:

app.add_route(SpecialClassView.as_view('special_class_name'), '/special_class_view')

Another idea would be to make the static file API similar to routing, with unique handlers:

@app.static(/static/images/<image_file>)
def image_file():
   ...

But these were out of scope for this request.

Fixes #336

[subyraman]

There's some issues with the CompositionView I want to clear up.

[subyraman]

Reopening this; the CompositionView testing issues are unrelated to this branch. Going to file another PR to add tests for the CompositionView (it has none, currently)

[r0fls]

This commit shouldn't be included: aa54785

[seemethere]

As always, super awesome work @subyraman