This patch adds a new mechanism for changing configuration options
without editing the configuration file, by sending options to running
instances through `alacritty msg`.
Each window will load Alacritty's configuration file by default and then
accept IPC messages for config updates using the `alacritty msg config`
subcommand. By default all windows will be updated, individual windows
can be addressed using `alacritty msg config --window-id
"$ALACRITTY_WINDOW_ID"`.
Each option will replace the config's current value and cannot be reset
until Alacritty is restarted or the option is overwritten with a new
value.
Configuration options are passed in the format `field.subfield=value`,
where `value` is interpreted as yaml.
Closes#472.
