Changing the key mapping for the Ack.vim search

- Creating a function, AckSearch, that search files
  using Ack.vim optionally filtering by file type,
  and prompts the user for continuing searches for
  terms smaller or equals to 3 characters
- Creating mappings to AckSearch
Removing NeoSnippet mappings
Moving Vim ultisnips mappings to the keymaps.vim file