Add support for FreeScout #95
Description
FreeScout is a solution that mostly is a copy of the old HelpScout v1, at least visually. It's also much cheaper.
FreeScout preserved compatibility with a lot of things that HelpScout did, including the header for signature verification, but it's named differently.
It's possible to easily adjust edd-helpscout plugin to work with both HelpScout and FreeScout.
This can be done in \EDD\HelpScout\Endpoint::validate() to make sure that the method listens to both possible variables.
I will submit a PR.