Change the default plugin directory for Neovim

And suggest users to call plug#begin() without an argument to avoid
confusion.

Allow cloning into an empty directory (junegunn#782)

Close junegunn#766

We can rely on git to check if the destination directory is empty.

New features:

  - Added `commit` option for fine-grained version control
  - Fixed issues with parallel installer on Windows and enabled it by default

Improvements:
  - Changed `PlugSnapshot` to create Vim script using the new commit hash support
  - `PlugDiff` shows pending updates for plugins locked on `commit` or `tag`
  - Enhanced output format of post-update hooks

Issues resolved:
  - Fixed Ruby installer to unshallowed tagged plugin on update (junegunn#350)
  - Fixed output format of Neovim installer (junegunn#340)
  - Remapped `q` in plug window to `bd` (junegunn#336)
  - Fixed freeze in Python installer (junegunn#318)
  - Case-sensitive validation of `on` arguments (junegunn#314)
  - Fixed post-update hook not to run on error
  - Fixed `for` option to load syntax files (junegunn#272)
  - Fixed UnicodeDecodeError from Python installer (junegunn#262)
  - `set nomodifiable` on commit preview window (junegunn#255)

New features:

  - Added support for Python 3.0+
  - Added support for Neovim Python
  - User autocmd triggered when plugin is loaded on-demand
  - Shallow clone by default

Improvements:
  - Updated for the new Neovim job control API
  - PlugUpgrade using git
  - Ignore case for "<Plug>"

Bug fixes and others:
  - Fixed &rtp missing on-demand loaded plugins when .vimrc is reloaded
  - Temporarily change &shellredir
  - Force evaluation of &statusline before running the installer
  - Explicitly specify format for `git show`
  - Fixed local plugin judgement on Windows
  - Changed the default number of threads on Windows to 1

Acknowledgements:
  - @starcraftman
  - @vheon
  - @itspriddle
  - @mhinz
  - @hori-ryota

- Python 2.6 compatibility

- Fix junegunn#184 - Avoid duplicate entries in g:plugs_order

New parallel installer using +python

- Thanks to @starcraftman

Bug fixes and improvements
- Fix junegunn#139 - Use git fetch+merge instead of pull
- Fix junegunn#145 - Merge duplicate on-demand loading triggers
- Fix junegunn#155 - `cd /d` on Windows
- Fix junegunn#159 by temporarily setting &shell to sh
- Use /bin/sh instead of /bin/bash (PlugSnapshot)
- Fix junegunn#162 - Do not use --progress option on git 1.7.0 or below
- Fix junegunn#154 - issues with paths containing spaces
- Fix junegunn#166 - v:version >= 703 || has('patch-7.2.374')
- Do not escape spaces in &rtp
- Remove `git::@` from URI if git 2.3.0 or above is found
- Add `o` key binding for PlugDiff
- Fix junegunn#177 - Check Python version >= 2.7
- Fix junegunn#174 - Update of tagged plugins
- Interpret the second argument to Plug command as tag option
- Fix junegunn#154 - error on windows when plug home contains spaces

0.6.2

- Fixed issues with NeoVim (junegunn#118, junegunn#127)
- Fixed to remove any existing ODL triggers (junegunn#130, junegunn#132)
- Fixed to check for full match of command name when setting ODL triggers (junegunn#135)
- Minor improvements and fixes (junegunn#123, junegunn#131, junegunn#135)

Minor bug fixes and improvements

- Fixed PlugUpgrade on Neovim (junegunn#111)
- Changed not to suppress messages from ftplugin when loaded on-demand (junegunn#112)
- PlugInstall or PlugUpdate will now install frozen plugins as well (junegunn#113)
- Fixed not to yield empty path elements in &rtp (junegunn#114)