Replace some QFN (12, 16 and 20 pin) with scripted ones

• This replaces the qfn-12, qfn-16 and qfn-20 footprints that had an easily find able datasheet attached with script generated footprints.
• It adds thermal via versions of these footprints.

Datasheets

• QFN-12-1EP_3x3mm_P0.5mm_EP1.65x1.65mm
• QFN-16-1EP_4x4mm_P0.65mm_EP2.7x2.7mm
• QFN-16-1EP_5x5mm_P0.8mm_EP2.7x2.7mm
• QFN-20-1EP_3x4mm_P0.5mm_EP1.65x2.65mm
• QFN-20-1EP_4x5mm_P0.5mm_EP2.65x3.65mm

Size definition added to script in commit https://github.com/pointhi/kicad-footprint-generator/pull/125/commits/edbfca6355e956ac3cf26ae97872c5e59fb82b70