Change escaping function to allow HTML

[kimcoleman]

In this line:
https://github.com/strangerstudios/pmpro-mailchimp/blob/dev/includes/settings.php#L79

The HTML is not being rendered and rather outputs:

Since v2.0, this plugin creates and synchronizes the <code>PMPLEVEL</code> and <code>PMPLEVELID</code> merge field in Mailchimp. <strong>This will only affect new or updated members.</strong>

I’d be ok with losing the <strong> and just use a printf to string in the PMPLEVEL and PMPLEVELID.

[kimcoleman]

Resolved in #144