Support incoming webhooks

[thesharp]

A feature request for a small built-in webserver to add routes for various incoming webhooks. A common use case scenario could be announcing GitHub commits/issues/PRs or travis-ci build resuts to the chat.

[pietroalbini]

I'm only worried the name webhooks will be easily confused with Telegram's ones. I'm thinking about referring it as the internal webserver, with an API like this:

bot.bind_webserver("127.0.0.1", 8000)

@bot.catch_http("/endpoint")
def catched_endpoint(request):
    pass