Introduce index-tag-all lfun

Introduce index-tag-all lfun

This is a convenience function for indexing. It adds a copy of the
index inset under cursor after any word in the buffer that is equal
(case-insensitively) to the word preceding the index inset.

The function can be easily used to generate bad indexes (and I warn
about that in the UserGuide), but if used with care, it can also be
extremely convenient.