Add match_prev_cmd strategy.

A new suggestion strategy 'match_prev_cmd' is available. This is a bit
more context aware variaton on the default strategy.
The suggestion will be:
- The newest history entry that matches the current prefix, AND
- Whose preceding history entry also matches the previously executed
command.

See src/strategies/match_prev_cmd.zsh for an example.

src/suggestion.zsh

@@ -19,3 +19,8 @@ _zsh_autosuggest_escape_command_prefix() {
 	# Escape special chars in the string (requires EXTENDED_GLOB)
 	echo -E "${1//(#m)[\\()\[\]|*?]/\\$MATCH}"
 }
+
+# Get the previously executed command (hookable for testing)
+_zsh_autosuggest_prev_command() {
+	echo -E "${history[$((HISTCMD-1))]}"
+}