proposal: allow setting default flag values for all commands.

[alexcb]

Use case

It can become tedious to specify the same flag for a command over and over, e.g. COPY --keep-ts ....

What if we created a new DEFAULT command, that would allow users to set the default hahaviours for each command, e.g.

VERSION 0.8

DEFAULT COPY_KEEP_TS=true

...

If a user has multiple Earthfiles in their project, a child Earthfile could then import defaults from another Earthfile, perhaps like:

VERSION 0.8

IMPORT --with-defaults ..

which would use the same defaults specified under the ../Earthfile.

Expected Behavior

flags would only have to be set once, in a single location.