Make verify command accept stdin input

I would like to make ``prmd`` more versatile, if you like it, I will expand the idea to the other commands.

This allow to pipe data in and out of the verify command, ensuring that errors are outputted on stderr.

For example:

	prmd verify < schema.json 2> errors

This also improve error formatting, by displaying file path along with errors,
allowing to filter errors per file easily.

For example:

    prmd verify schema/ |& grep account

This will also allow to use ``Prmd.verify()`` outside of the cli.