Add option to force-override "Reply-To" field

[erommel]

Tested the plugin with some other plugins that can modify "Reply-To" address and figured some of those can modify reply-to field unexpectedly.

Solution: add "Force Reply-To" checkbox in the settings. If it's enabled, the plugin removes all the "Reply-To" addresses before adding own.

Also make sure the plugin is processing "phpmailer_init" hook the latest, in order for some other plugins to not be able to override any SMTP settings.

[erommel]

Lowered "phpmailer_init" hook priority d051ae9

[erommel]

Confirmed this is needed, since even eStore plugin modifies this value :) Would be a good idea for the user to decide whether he wants that setting forced or not.

[Arsenal21]

ok