Handle child nodes without modifying attributes

(To be further tested and commented.)

Limits substitution to text nodes only, solving the problem of HTML attributes being replaced. For example:

A sentence with Proper Nouns and a <a href="http://example.com" title="Example Corp.">link</a>.

Previously, "Example" and "Corp" would be surrounded by span tags, breaking the link tag.

Since substitutions are made recursively, this could be quite slow if care isn't taken in which element to pass to tame() when calling the plugin.