Updates to include Discord without additional deps, make smtp optional

[phybros]

Added the ability to send discord messages via webhooks (no new dependencies were added). Configured using a new [discord] section in the config file (I updated the example file as well).

Also made smtp disableable using a new enabled = false flag in the [smtp] section since people may not want email at all, and it writes an ERROR log if you don't have the smtp host set up.

[phybros]

Hey I know it's been two years but do we think this will get merged? 😅

[Chronial]

Hey, sry for being so inactive here. I'm still against merging specific notification services directly and would rather see a plugin system: https://github.com/Chronial/snapraid-runner/issues/49