gbz80disasm: data detection

the is_data flag never did anything. now it does

encountering the end of a branch with outstanding labels will read anything between said labels as data
gbz80disasm: detect data tables referenced in asm
gbz80disasm: local/global address conversion + fix find_label
gbz80disasm: fix data handling

get the scalpel out of there
gbz80disasm: just use lowercase labels
Merge pull request #153 from yenatch/gbz80disasm-data

data detection in gbz80disasm