Add shortcut for known ports

[tigoe]

typing out board attach /dev/cu.usbmodem1421 mySketch could be simplified to board attach 0 mySketch would be one faster way. tab-autocomplete works, for now you could just add that to the readme.

[ubidefeo]

Issue closed because autocomplete is implemented
https://arduino.github.io/arduino-cli/0.27/command-line-completion/