BibConvert: small fixes to LIM/LIMW/WORDS

• Wrote test cases for LIM, LIMW, WORDS formatting functions.

• Fixed documentation for these functions. Clarified left/right trimming and fixed wrong L/R direction in the WORDS example.

• Fixed the code of LIMW in case of multiple matches of the needle and in case of an empty needle.

• Now the behaviour of LIM, LIMW and WORDS is consistent with the documentation and examples.