Add support for generic webhooks

[nijel]

This can be useful with push notification services such as Gotify.

Built on top of #95

[coveralls]

Coverage increased (+1.4%) to 76.667% when pulling d556c04 on nijel:webhooks into 5b9a356 on mtakaki:master.

[mtakaki]

It looks great! Thank you so much for the awesome work. I added a few comments.

[mtakaki]

Here is an overview of what got changed by this pull request:

Coverage increased per file
===========================
+ cachet_url_monitor/configuration.py  3
         

Coverage decreased per file
===========================
- cachet_url_monitor/scheduler.py  -2
         

Complexity increasing per file
==============================
- cachet_url_monitor/configuration.py  1
- cachet_url_monitor/webhook.py  2
         

See the complete overview on Codacy

[mtakaki]

Thank you once again for this awesome contribution :)