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 output 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.