Add config option to customize User Agent and other headers

[Kade-N]

Just a value at the top of the config where a User-Agent can be specified. Currently it always uses one that mimics a browser.

By default it should probably say the plugin name, server name, and maybe IP of the server.

Variables should be supported in this.

[Kade-N]

On second consideration, it would be more useful to be able to set any list of headers (including User-Agent) both at the top of the config, and individually in each webhook (the latter overriding the former)