Command line args to make local running easier

[byronrthomas]

It can be tricky to use routemaster locally without hacking the config because:

• webhooks are contained in the config and typically point to production endpoints (e.g. https://www.example.com), so in order to avoid sending bogus requests to production services you typically find-replace in the config e.g. https://www.example.com -> http://localhost:8000
• certain logger pluggins e.g. prometheus / sentry also assume that routemaster runs in a production-like setting - you typically remove these loggers from the config in order to run locally

This change adds command line options & associated config to workaround these issues and make running locally possible without hacking config in these ways, specifically:

1. add a command-line option --local which looks for a newly added localport setting in the webhooks config items
   • when --local is set and the webhook called matches a given webhook config that has a localport setting, the URL is overridden to use HTTP scheme and a netloc of 'localhost:{0}'.format(x) where x is the value of localport in the config
2. add a command-line option --python-logging-only which skips instantiation of any loggers that are not the basic routemaster PythonLogger, so the advanced loggers will not be used

This has been tested manually. I'm not sure that the webhooks config is the ideal place for this - it seems like there may be an alternative that's higher up the config tree and a bit more specific to this case. Also, I'm open to suggestions for alternative names of these options if people don't think the names are helpful.

[coveralls]

Coverage decreased (-0.6%) to 99.257% when pulling 98c576c on local-usage-improvements into 17db5ac on master.

[danpalmer]

Will re-open if we think this is useful when working on Routemaster next, but I think there might be a better way of doing this, potentially with Peter's changes to the config system.