Version 0.5.6

- Added `g:plug_window` (#57)
- Changed URL for `PlugUpgrade` (#58)
- Added `g:plug_url_format` (#62)
- Fixed PlugDiff with git config `pull.rebase=true` (#64)
- Fixed commit review of `PlugDiff` when directory changed on BufEnter/BufLeave (#65)
- Changed not to print message after PlugUpdate when there was no update (#66)