Version 0.5.5

0.5.2:
- Added `PlugInstall!` and `PlugUpdate!` for forced-run of post-update hooks

0.5.3:
- Added `-bar` option to the commands so that they can be written one line
  with BARs. e.g. `PlugClean! | PlugInstall`
- Fixed `PlugUpgrade` so that a restart (or reload of .vimrc) is not required
  even when a very old version of vim-plug is upgraded
- Removed redundant code for creating of `g:plug_home` directory

0.5.4:
- Added plug#helptags()
- Added plug#load(name...)
- Updated `:PlugStatus` to show which plugins are not yet loaded
    - You load them with `L` key (visual range allowed)
- Updated `:PlugDiff` to allow reverting the update with `X` key

0.5.5:
- Use `git::` prefix to avoid password prompt on git 1.7 (#56)
- Allow Plug command even when git executable is not found (#52)