add back g:indexed_search_keep_cursor_on_initial_result

Unfortunately, unlike what 206137d proposed, just a
    `nmap * *N`
in your `.vimrc` won't work because plugins are sourced after it and they
would overwrite any mappings you had.

Add back this option which makes the cursor stay on the word you *.
It's renamed to `g:indexed_search_dont_move` though.
Simplify the mappings

Hope it doesn't break anything!
Change some comments
Add option g:indexed_search_unfold

Like `g:indexed_search_dont_move`, it's there to overcome the fact that
a normal mapping, like `nmap n nzx`, won't work with the plugin.  Like
it says, it opens just enough folds so that you can see your search
result, while keeping every other fold, opened or not, intact.