Allow skipping completion suggestions when buffer matches a pattern

Set ZSH_AUTOSUGGEST_COMPLETION_IGNORE to a glob pattern to have the
completion suggestion strategy never make suggestions when the buffer
matches the pattern.

This can be helpful when some completion routines you have are
particularly expensive and you want to prevent them from running
automatically on every keystroke.

See GitHub issue #463.

src/strategies/completion.zsh

@@ -96,6 +96,12 @@ _zsh_autosuggest_capture_completion_async() {
 }
 
 _zsh_autosuggest_strategy_completion() {
+	# Reset options to defaults and enable LOCAL_OPTIONS
+	emulate -L zsh
+
+	# Enable extended glob for completion ignore pattern
+	setopt EXTENDED_GLOB
+
 	typeset -g suggestion
 	local line REPLY
 
@@ -105,6 +111,9 @@ _zsh_autosuggest_strategy_completion() {
 	# Exit if we don't have zpty
 	zmodload zsh/zpty 2>/dev/null || return
 
+	# Exit if our search string matches the ignore pattern
+	[[ -n "$ZSH_AUTOSUGGEST_COMPLETION_IGNORE" ]] && [[ "$1" == $~ZSH_AUTOSUGGEST_COMPLETION_IGNORE ]] && return
+
 	# Zle will be inactive if we are in async mode
 	if zle; then
 		zpty $ZSH_AUTOSUGGEST_COMPLETIONS_PTY_NAME _zsh_autosuggest_capture_completion_sync