Simplify combine command

This is the same idea as #24, applied to combine. I start with the easy parts.

This allow to use ``Prmd.combine`` outside of the cli, it makes less assumption on
where file suppose to be or where they should be written.

This also allow to pipe prmd commands:

    prmd combine schema/ | prmd verify

And have more flexibility on output:

    prmd combine -o schema.json schema/

And you can still diff the file using diff tools:

    prmd combine schema/ | diff schema.json -