Drag and Drop Support

Allows dropping a single file onto ynabConvert.py to produce an
importable YNAB file.
If run without arguments or more than one file is dropped then falls
back to 'EXPORT.CSV'.
Revert "Drag and Drop Support"

This reverts commit 9a861709ec73101a8f842c87b2683c069941b121.
Drag and Drop Support

Allow exactly 1 file to be dropped onto 'ynabConvert.py'.

If run with 0 or more than 1 arguments then fall back to using
'EXPORT.CSV' for input file.