Allow pressing 'q' to abort the running tasks, then 'R' to retry (jun…

…egunn#1289)

If you press 'q' while `PlugInstall` or `PlugUpdate` is in progress,
the current version of vim-plug immediately closes its window and aborts
all the running processes.

This commit enhances the 'q' map so that it doesn't close the window,
but instead displays the aborted tasks so that the user can press 'R' to
retry the tasks.

Fix junegunn#1285

Open vim-plug window in a new tab (junegunn#1274)

* Open vim-plug window in a new tab not to disrupt the current window layout
* Open preview window on the right to better show the diff

If you prefer the old layout, use the following configuration:

    let g:plug_window = 'vertical topleft new'
    let g:plug_pwindow = 'above 12new'

Reset &rtp before 'do' to invalidate Neovim cache of loaded Lua modules

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