Sanitize Content Handler

This pull request adds the ability to configure and run the Sanitize Content Handler by editable element tag name, in addition to element ID and class attribute functionality that exists today.  

Additionally, the Sanitize Content Handler initialization and setup has been refactored to cache Sanitizers per config.  Also, the relevant section in the plugin_contenthandler guide page has been cleanup up to match released implementation plus this enhancement.

This code has been tested across browsers by our QE.

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