add support for multiple codon tables

[gpertea]

Originally posted by @Maarten-vd-Sande in gpertea/gffread#21 (comment)

Just dropping in to say that supporting multiple tables would be an awesome feature (for me). I haven't found any other tool that works as easy and fast as gffread.

Would you support a PR in this direction, if so, can you give some pointers? It's been over 5 years since I did anything serious with C++ so it might not result into anything useful though 😇 .. I can't seem to find where the table is defined..

gclib/codons.cpp

Line 7 in f6817ea

static char codonData[]={ //long list of 3+1 characters (codon+translation)