Cachet-url-monitor

As part of 99, cachet-url-monitor now supports different token providers. It included refactoring, so it can be easily extended to support new sources.

Still backwards compatible with old configuration files.

Thank you nijel for the great new additions!

- Removing unused token from `Configuration`.
- Adding support to webhooks.
- Improving incident handling with customizable incident message.

Thanks to nijel, we got numerous bug fixes:
- Removing usage of scheduler and relying on thread sleep.
- Incorrect location of latency unit.
- Fixing the metrics push that was pushing it twice.

Accidentally introduced a bug when moving code from `configuration.py` to `scheduler.py` (81). Kindly fixed by chris-str-cst through 82.

Fixing bug introduced in 0.6.2 when a `CachetClient` class was created and it was preventing it from updating component status. It wasn't calling `ComponentStatus.value`.

Upgrading dependencies:
- PyYAML 5.3

Dev requirements:
- coverage 5.0.3
- coveralls 1.10.0
- mock 3.0.5
- pytest 5.3.3

And finally fixing the code coverage metrics, that was broken due to running pytest with the installed package, rather than the source code.