Create an email plugin

[gigxz]

• Add a plugin that has an action for sending an email. This is for the Agreement Engine https://github.com/metagov/agreements-prototype.
• Metagov already has a Mailgun account that we use for our Discourse server, so maybe we should use that. Mailgun: https://documentation.mailgun.com/en/latest/api-sending.html#sending
• Plugin tutorial: https://docs.metagov.org/en/latest/plugin_tutorial.html

[lukvmil]

added in pull request #72

[lukvmil]

Looks like there were some errors in the build, I will look into it

[lukvmil]

Merged into master, closing this issue!