Fix NextIndToken and PrevIndToken

In bfeaae1d5b628ceb479cf29efbd44f9bde1351b1, they returned an empty list, but
that was not checked by the callers.
(#2) Fix multiple "-spec" clauses (if there is no "when")
(#24) Add "Indentation from shell" to README

Closes #24.
helper.vim: Delete misleading comment

In 2de159679d519b43041f8212713085ef511caf51, I removed the line to which the
comment referred, but I forgot to remove the comment itself.