Add option to setup settings on wp-config file

[planetahuevo]

Hi, I would like to request this feature. There are many advantages for this, some of them are:

• Secure your API KEY data. The admin dashboard can be accessed for many users, and sometimes you do not want that data to be leaked to your developers, clients or the SEO guy.
• It allow to easily deploy websites with your plugin using scripts. No need to go into the UI at all.
• It prevents clients or other admins to delete the API key by mistake, affecting the delivery of the emails, which is something critical on any website, as you know. :)
• It allows to use the POSTMARK_API_TEST key programmatically with no need to add extra functionality to the plugin for that. This was part requested on #10
• Following up that, this will be really useful to add this into staging sites that are generated by scripts, so email won´t be send by mistake when testing upgrades (a common thing in WordPress).
• It will facilitate to convert this plugin into a Must Use plugin, as requested on #5 (and #3)
• It will also help to make this work with a multisite installation in the future, as settings can be used for all installations by default. #11 and #15
• It will make sense if you decide to complete #33 and add wp cli support, because you can ignore the UI completely.

So, I think it is clearly something not that complicated that will bring a lot of benefits to everyone. I hope you can consider this. Best regards and thank you for the great service and support you provide!