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 d556c04111d8dc7a8358661545e1c40694e057a8 on nijel:webhooks into 5b9a356f2dda96b5d8d5bd0b54818543c8e9012b on mtakaki:master.

[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 :)