rename `region` argument to `include`

…for symmetry with `exclude`

thanks to @ivyreyes for the suggestion

TODO: add `region` as an undocumented alias to `include` for backward
compatibility, at least temporarily
allow a single argument to be passed to the `new` constructor

…and treat it as the value of `include`
support `region` argument as alias to `include`

…for backward compatibility
when `include` is the first argument, the `include` key is optional
update changelog
improve BUILDARGS handling of `region` and `include`

thanks to feedback from @aarondcohen
dev v0.01_01