giffl-pi region input mode requires improved flexibility

[calizilla]

Running giffl-pi --giffl_input chr21 --gtf <gtf> --label <label> produces an empty BED file.

Running giffl-pi --giffl_input chr21:1-46,709,983 --gtf <gtf> --label <label> fails with ERROR: invalid GIFFL input chr21:1-46,709,983

Add flexibility to allow for whole-chr and commas in start/end