Switch preferred parts to JSON format

Switch preferred parts to JSON format

This should make the file a bit easier to add fields to where
necessary and provide some validation during use.

To search the file for a string, use the new 'pgrep' command
in bin:

    $ bin/pgrep SAMD21G17D preferred.json
    {
	    "device": "IC",
	    "footprint": "vqfn-48",
	    "loadstatus": "smt",
	    "mfg_part_number": "ATSAMD21G17D-MUT",
	    "provided": "yes",
	    "value": "SAMD21G17D",
	    "vendor": "mouser",
	    "vendor_part_number": "556-ATSAMD21G17D-MUT"
    }

Signed-off-by: Keith Packard <keithp@keithp.com>