Sanitize $pretext output for fun and profit

[fulltrucker]

It might be good to sanitize the output of the $pretext variable, for best practices and because we don't want malicious code to sneak in. Presumably it's "okay" to do it the way I have it, since the input field is only accessible if a user is logged in to the Admin plugin, but we should sanitize it anyway.

https://learn.getgrav.org/security/developers
http://php.net/manual/en/filter.filters.sanitize.php