Adds the option to skip TLS verification for a Gotify instance

[tammert]

Fixes #382.

Tested this with the following Gotify server using a self-signed certificate:

docker run -d -p 443:443 -v /tmp/gotify:/app/data -e GOTIFY_SERVER_SSL_ENABLED=true -e GOTIFY_SERVER_SSL_CERTFILE=/app/data/localhost.crt -e GOTIFY_SERVER_SSL_CERTKEY=/app/data/localhost.key gotify/server

With the new flag to the instance as described above:

$ ./watchtower --trace --notification-gotify-url=https://localhost:443 --notification-gotify-tls-skip-verify --notification-gotify-token=AL8lZtthYtxzSgw --notifications=gotify -R
INFO[0000] Running a one time update.
DEBU[0000] Checking containers for updated images

Without the new flag:

$ ./watchtower --trace --notification-gotify-url=https://localhost:443 --notification-gotify-token=AL8lZtthYtxzSgw --notifications=gotify --notifications-level=info -R
INFO[0000] Running a one time update.
Failed to send Gotify notification:  Post "https://localhost:443/message?token=AL8lZtthYtxzSgw": x509: certificate signed by unknown authority
DEBU[0000] Checking containers for updated images

And, of course, the notifications were received when the new flag was active :).

[simskij]

Thank you for the contribution! The only check that's failing is godacov, so I'll go ahead and merge this regardless!