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

Prevents some flakiness in tests

Based on e702ec4697/async.zsh (L400-L406)

Checks whether `zpty` gives a file descriptor, which was not the case in
older versions of zsh.

Based on a4b2f81c96/async.zsh (L395-L401)

As far as I know, `fc` makes it impossible to tell whether history items
used an actual newline character or the string "\n". Pulling from the
`$history` array gives a more accurate representation of the actual
command that was run.