Simplify, make more generic, yet break backward compatibility.

-D TARGET="psql" can customize the full psql command, including the former CMT_TARGET_PSQL_URI.

When variables are defined via -D, the CMT_ prefix is added automatically.

clunky-migration-tool -m filetree:debug -D TARGET_VERSION=v3 v5

The above passes CMT_TARGET_VERSION=v3 to the debug script.

v0.0.1

Update docs.