Allow Postmark-wordpress to be used as a drop-in plugin.

[atheken]

Allow Postmark-wordpress to be used as a drop-in plugin, see here: https://codex.wordpress.org/Must_Use_Plugins

[mgibbs189]

Duplicate of #5?

[guillaumemolter]

Yes definitely a duplicate of #5. Drop-ins are only for very specific type of features (mostly caching, DNS, advanced DB management etc...). Must-use plugin is the way to go as wp_mail is a pluggable function

[cfoellmann]

CLOSE because of duplicate #5