Trim whitespace.
Add support for WP Better Emails.

Note - Requires my forked version of WP Better Emails at the moment until
my changes to WP Better Emails are added to the official plugin:
https://github.com/r-a-y/WP-Better-Emails/tree/changes

WP Better Emails wraps the original plain-text email content around a HTML
template; this interferes with the positioning of RBE's marker and how RBE
parses the reply.

This commit repositions the RBE marker in HTML emails created by WP Better
Emails so the RBE marker is at the beginning of the HTML body.