ENH: Allow specification of tag delimiter

Users can now specify tag delimiter in the settings file (defaults to
comma) and separator (defaults to comma plus space).  If the delimiter
is present in the tag, then the tag is surrounded by quotation marks.

This is a small API change from before (when rendering existing tags,
the quotation marks used to be added to tags with spaces; however, this
is uncessary so I think it's best to add quotation marks only when the
tag contains the delimiter).  This makes the rendering more uniform in
the UI (e.g. when users are adding new tags, they may not always add the
quotation marks and might be surprised when the save and then go back to
edit the form and see quotation marks have been added).