Allow for manual triggering of GS for table

[IanMayo]

As authors mature in their use of G-S, they may wish to control the level of integration into their content.

This could include

• specifying that GS is to be enabled for a table
• specifying enrichments for specific rows and/or columns

Note: One of the upstream technologies (DITA-OT HTML transform) that we may be using does not allow the specification of data- tags in HTML content. So, we have to use classnames to do this. I recommend we have gs- at the start of the tag-name to avoid polluting content.