Improvements to table binding with existing content

When a <table> element already exists on the page, it would be useful to utilise the contents of the table, retaining parts of it as a template when binding.

This minor release introduces improvements to help this case:

• Existing <th> elements that don't match any bound data are preserved.
• Bound data that doesn't match any existing headers is discarded.
• Importantly: existing <tr> elements can be marked with data-template to add pre-templated row shapes for advanced designs.