Fix bug in OpenSelection(). Access b:var after changing buffers.
Fix bug in OpenNifflerBuffer(). No quotes around bufname
Add call to startinsert! in OpenNifflerBuffer and RedrawScreen
Replace call to system `find` command with `ag` command.

- Reducing the size of the candidate list is more important than building the
  list quickly
Modify RedrawCandidateList to use b:last_prompt optimization
Replace vglobal filtering with call to `find` command.

- Replace `ag` command with `find` command again as the default filtering
  command.
Remove RedrawCandidateList(), b:last_prompt, & b:file_list

- Unnecessary now that entire list is cleared and replaced with results of new
  find command.
Fix bug in FilterCandidateList(). Can't delete 2,$ if only 1 line
Add insert mode movement mappings
Reorganize functions