This allows an escape code to invoke user-defined extensions found in
/home/rahm/.local/bin/alacritty-ext.sh. The terminal passes to this
script the escape-code arguments.

This allows things like opening links in the browser even through an ssh
connection.

The config values are hardcoded atm.

This adds the `\` character to the list of characters which will
terminate matches in Alacritty's default hint for URL recognition.

This patch removes the `hold` option on `alacritty_terminal` in favor of
a `drain_on_exit` option, which will drain the PTY before shutdown. The
hold logic is instead handled in `alacritty`.

The daemon without socket is not that useful.

This fixes a crash with partial utf8 input and also speeds up parsing
in some cases.