Change a `#each` + `#<<` to a `#map`

Map better represents our intentions here, so use it.
Clear statusline upon closing the match window

Originally submitted at:

  https://github.com/wincent/Command-T/pull/51

Relevant comment from the discussion:

  Had a closer look at this. This can only be reproduced under very
  specific circumstances: when running Vim in a 256 color terminal
  within tmux using a color scheme that uses a non-black background
  color the statusline never clears unless this patch is applied. For
  example, using using wombat256 or gruvbox
Update HISTORY section in docs
Merge branch 'ton/clear-statusline' into next

* ton/clear-statusline:
  Update HISTORY section in docs
  Clear statusline upon closing the match window