The engine now unconditionally emits a new type of event, a
PreludeEvent, which contains the configuration for a stack as well as
an indication if the stack is being previewed or updated. The
responsibility for interpreting the --show-config flag on the command
line is now handled by the CLI, which uses this to decide if it should
print the configuration or not, and then writes the "Previewing
changes" or "Deploying chanages" header.
The existing logic would flow colorization information into the
engine, so depending on the settings in the CLI, the engine may or may
not have emitted colorized events. This coupling is not great and we
want to start moving to a world where the presentation happens
exclusively at the CLI level.
With this change, the engine will always produce strings that have the
colorization formatting directives (i.e. the directives that
reconquest/loreley understands) and the CLI will apply
colorization (which could mean either running loreley to turn the
directives into ANSI escape codes, or drop them or retain them, for
debuging purposes).
Fixes#742
