Feature: Plain-text template

Hi Nicolas,

Got your email.  I've added my changes to this pull request.

The main changes are:
* Adding a plain-text template option on the admin settings page
* Making sure we can use our plain-text template when sending - check out the `send_email_template()`, `send_html()` and `validate_options()` methods
* Adding two filters to the plain-text and HTML body - ` 'wpbe_plaintext_body'` and `'wpbe_html_body'` - this will be useful for plugin devs so they can modify the email contents before sending
* Formatted the code to use WP's coding standards - https://codex.wordpress.org/WordPress_Coding_Standards

Let me know if you have any questions!