Add diff --external. Runs external diff command.

[fujiwara]

Add --external option to diff command.

ecspresso diff invokes the external command with two arguments, "remote" and "local" resource files that are rendered.

The command should exit with status 0. You need to write a wrapper command if it exits with a non-zero status when two files differ (for example, diff (1)).

$ ecspresso diff --external="difft --color=always"

$ ECSPRESSO_DIFF="difft --color=always"