add external clock source

[aster94]

hi @bperrybap and @PeterVH
based on the above schematics of the cheap usbasp

looks like that there are some extra pin connected to the 10-pin connector. It would be useful to have one of these pins output a clock source (1mhz, 8mhz or 16mhz). Adafruit and Nick Gammon did the same in Arduino as ISP, please have a look: https://forum.arduino.cc/index.php?topic=112442.0