Improve completion suggestions

To prevent the suggestion from not starting with the buffer string.

Example:

`ls / /[cursor left][cursor left]b`

Before the patch, suggests `ls /b /ls /bin/ /`

After the patch, suggests `ls /b /bin/`.

https://github.com/zsh-users/zsh-autosuggestions/issues/343#issuecomment-401675712

If running in sync mode and a completion takes a long time, the user can
^C out of it. Without this patch, the pty will not be destroyed in this
case and the next time we go to create it, it will fail, making the
shell unusable.

Just insert the first completion directly into the buffer and read the
whole buffer from the zpty.

Add install directions for Antigen

Fixes/no zpty

It is not available in zsh versions older than 5.3

Not needed after move away from zpty for async

Features/completion suggestions

Error looked something like:

```
% echo 'f(zpty):8: unmatched '
_zsh_autosuggest_capture_completion:zpty:9: no such pty command: zsh_autosuggest_completion_pty
_zsh_autosuggest_capture_completion:zpty:14: no such pty command: zsh_autosuggest_completion_pty
_zsh_autosuggest_capture_completion:zpty:21: no such pty command: zsh_autosuggest_completion_pty
```

According to `man zshmodules`, the args to `zpty` are "concatenated with
spaces between, then executed as a command, as if passed to the eval
builtin." So we need to escape the `$` so that `$1` is passed to eval
instead of the value of `$1`.

https://github.com/zsh-users/zsh-autosuggestions/issues/341

See http://www.zsh.org/mla/users/2018/msg00432.html

Refactor async mode to no longer use zpty

See technique used in `fast-syntax-highlighting`:
- ca2e18bbc9
- http://www.zsh.org/mla/users/2018/msg00424.html

Fixes #337

`zpty -r` with a pattern seems to have some funky behavior on older
versions, giving unpredictable results

Need to make sure compinit is called in the pty or the shell hangs

Only a problem in synchronous mode

The `zsh -f` running in the PTY doesn't know about the non-exported
variables and functions defined in the original shell, thus can't make
suggestions for them. Run local functions in the PTY instead of a new
`zsh` process.

We have to handle things differently based on whether zle is active or
not (async vs. sync mode).

Only need the first completion result

Uses https://github.com/Valodim/zsh-capture-completion with some slight
modifications.

`stty` occasionally hangs (always in CircleCI) inside the async pty.

Disable the tests for now until we can figure out and fix/workaround
this issue.

v0.4.3

For unknown reasons, the pty will occasionally quit running. In these
cases, we still want to remove it so that a fresh one can be created. We
don't actually need this check because error messages from `zle` and
`zpty` are redirected to /dev/null.

One sure way to kill all currently running pty's is to run `exit` in a
subshell. Even without zsh-autosuggestions loaded, the following works:

    % zmodload zsh/zpty
    % zpty -b foo cat
    % zpty -b bar cat
    % zpty
    (31689) bar: cat
    (31666) foo: cat
    % $(exit)
    % zpty
    (finished) bar: cat
    (finished) foo: cat

Issue #188. PR #324.

Thanks to @toadjaune and @IngoHeimbach.

Specific case where this matters is following:

Be in vi insert mode with some text in the buffer and the cursor at the
end of the buffer. Press `esc` to trigger `vi-cmd-mode widget`, then
before the cursor moves (KEYTIMEOUT), press `h` to trigger
`vi-backward-char` widget. When `vi-cmd-mode` original widget exits,
KEYS_QUEUED_COUNT will be non-zero and the suggestion will be lost.