Add support for Fluxer to reporting destinations

Fluxer is a Discord-like app that accepts Discord webhook formatting, yet doesn't work when webhook URLs are put into the text-field.

PatchMon believes it's a standard webhook and when testing, the test notifications never go through.

Fluxer uses the following URL format: https://{ur}l/api/webhooks/{ID}/{token} just like Discord.

Please consider allowing Fluxer as an alternative / swappable URL type, or implementing Apprise (As they support Fluxer, and may more other options)

Thank you,

Jacob W.