Add --locus_id_prefix to convert_gff3_to_gbk.py

Add a new (and optional) command-line argument to convert_gff3_to_gbk.py, --locus_id_prefix. Whatever string is passed to this option will be prepended to the GenBank LOCUS id for each assembly/sequence in the input GFF3. Note that, when used in conjunction with --output_dir, this option will also affect the names of the output GenBank flat files. The main purpose of this option is to provide an easy way to avoid (file and sequence) name conflicts in the case where the same sequence (e.g., "scaff00001") is present in multiple input genomes. Certain Ergatis components, for example, will fail if the set of input sequence ids and/or filenames is not unique.