command and conquer them all, nod if you're happy

This release mainly adds process priority settings and a couple of shorthands for other functionality.

Features

• New silent parameter disabling all logger calls except of logging.DEBUG levels
• New on_exit parameter that takes a callback function as argument, convenient when used in a thread
• valid_exit_codes now accept boolean True which means "all" exit codes
• New priority parameter which sets cpu bound process priority to 'low', 'normal' or 'high'
• New io_priority parameter wihhc sets io bound process priority to 'low', 'normal' or 'high'

Fixes

• Fix output capture failure should be an error log instead of debug
• Fix no longer show debug logging for stdout or stderr when empty