Add decode symbol helper and flex option

[zuckschwerdt]

This adds a helper to decode symbols.

I.e. for a code of {25}7d577d8, which is 01 11 11 01 01 01 01 11 01 11 11 01 1
using symbols of 01 and 11 the result is {12}616, which is 011000010110

Available as flex decoder option, note that the symbols bits need to be MSB aligned:
symbol_zero={2}4,symbol_one={2}c