Generalize the --encoding-xlsx input option for other file types in in2csv

[dreavjr]

I've recently tried to use in2csv in a GIS dBase IV dbf file and was unable to do so until I amended (in a very ugly way) in2csv.py to get an additional --encoding-dbf command-line argument (which mirrors --encoding-xls in passing an encoding argument to agata.Table).

Since probably this is not a very promising solution (I imagine in the future an --encoding-xxx for each possible data file), wouldn't it be better to just have an --input-encoding parameter? Would that break anything else?