simplify combine command

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 -
Merge pull request #25 from heroku/simplify-combine

Simplify combine command