Search for a List item based on its string

If the List has vi mode enabled and a `search` function is set on the options
property then hitting `/` will call this function passing in a callback.
The search function will need to call the callback with the search string
Added docs on the new `search` option for List's
Added a new function to do a regexp match on ritems
Merge pull request #54 from bulkan/feature/list-search

Run a `search` function in vi mode for List's