add feature_dbxref to flybase tables

i need the feature_dbxref tables from flybase (nif-0000-00558) in order to access the gene synonyms needed for the KEGG data.

for example, feature id 3200709 has a dbxref of 219898, which is for the label 'CG10161' of db=1.

also, should put any identifiers from this feature_dbxref into the id mapping table.

this should be as simple as adding a new item to the fetching script, and then adding a new interface in the interop xml.