OpenXML: Allow excel metadata to be extracted as <note> instead of <context>

Suggested by Jonathan here:

  1. (Excel filter) The extracted metadata are currently stored in the XLF under the <context> element. The problem is that some CAT tools (such as XTM) expect comments/metadata to be stored in the <note> element instead. By default it just ignores the <context> element altogether. It would be great if the user could define the XLF element name where the metadata should be stored, or perhaps have a choice between the 3-4 most common options, so that this metadata parameter could be used in conjunction with all TMS/CAT tools.

This is sort of the inverse of an option in the Table/CSV filter ("Expose comments as metadata"), so it's still a feature-parity thing.