Implement Previous/Next Entry links within tag. Fixes #943

Building off of the existing code base for the generic previous/next
entry links.  The new links use the same structure, but pass in one
additional argument (the tag to search within) to go.bml. If a tag is
passed in, it narrows the SQL query in get_item_id_near2()
appropriately.
Limiting display of tag links. Update to #943

Displaying the previous/next links everywhere is a bit overkill. Added
logic to limit to only show on single-entry pages, and /tag/foo
multi-entry pages.
Merge pull request #947 from woggy/feature/2014-tagnav

Add Intra-tag navigation links - fixes #943