Reset key combination

This adds a `g:clever_f_reset_key` that you can set to reset the current search. Which allows actions such as `fr;fp` to move to the first occurrence of `r`, reset the search with `;` (if that's your key of choice) and move to the first occurrence of `p`. I also added this option to the readme. I spent some time writing tests for this but like `g:clever_f_not_overwrites_standard_mappings` since this is defined based on existence on load it cannot be tested afterwards with vspec.

I also added guard-shell and terminal-notifier (if you're on OS X) to the Gemfile for testing. The build will probably fail since `to_be_mapped_to` was removed from your matchers [here](https://github.com/rhysd/vim-vspec-matchers/commit/b5af582eca1220b12ba2bb160bb720c19d9d6688)