escape pattern-matching chars on $prefix for match_prev_cmd strategy

Maybe this is also a fix for #247, #248 and #258. Supersedes #267.

Testcase:
Using match_prev_cmd strategy and with these lines in history:
echo '1^'
echo '2^'
echo '1^'

type:
echo       (unexpected suggestion echo '1^' instead of echo '2^')
echo '1^1  (wrong suggestion echo '1^1echo '1^')
echo '1^#  (error "bad math expression")

replace tabs in Rubocop config with spaces

YAML does not permit tabs and many parsers will error when loading a tab-indented file - http://www.yaml.org/faq.html

Github #247 and #258

This commit adds the 'emacs-forward-word' widget to the list of partial
accept widgets.

Update README.md

Added installation instructions for AUR, Homebrew

v0.4.0

While they are still experimental

Fixes/bpm async fix

[GitHub #219]

Intended to be helpful for folks using bracketed-paste-magic and other
widgets that use `zle -U`.

Depends on patch to ZSH from workers/40702:

  http://www.zsh.org/mla/workers/2017/msg00414.html

Puts in a better fix for #126 and related issues.

Can attach while tests are stopped with `tmux -L <socket_name> attach`

Set up Circle CI

Fixes flaky test

There's no need to re-run feature detection or async_start on every
precmd. Just do those once.

Specifically, highlighting breaks if our widgets are wrapped by z-syn-h
widgets.

Should fix #126

So that it doesn't conflict when the file is sourced again

These options have been deprecated for over a year.

It seems like all the zle-* methods are special and shouldn't be
monkeyed with.

Specifically `zle-isearch-update` and friends. Binding that widget
caused `history-incremental-pattern-search` to stop working.

Fixes zsh-users/zsh-syntax-highlighting#387

Asynchronous suggestions