Trim whitespace.
Add headers to options page.
Add plain-text template option to settings page.
Switch 'wpbe_help' element ID to class so multiple links can toggle the contextual help tab.
Capital P for 'WordPress'
Add default plain-text template option.
Use spaces instead of tabs in template_vars_replacement() method.
Make set_email_template() method more flexible.

* Adds a second parameter so we can use different templates later on.
* This will be used so we can set our plain-text template during sending.
Modify send_html() method to include our plain-text template.

* Use our modified set_email_template() to set the 'AltBody' variable with our plain-text template
* Add filters to the plain-text body and HTML body so plugins can alter the contents of the email before WPBE sends out the email.
Add plaintext_template_editor() method. Used on the admin options page.
Modify validate_options() method

* Validates 'plaintext_template' option
* Formats method with whitespace as per WP coding standards - https://codex.wordpress.org/WordPress_Coding_Standards#Space_Usage
More minor WP coding standards changes.