Fix download of 2010 general Linn County precinct results

The 2010 general election precinct-level result file for Linn
County inexplicably has an "xlsx" file extension while the other
files have an "xls" extension.

Addresses https://github.com/openelections/core/issues/203
First pass at Iowa 2010 General Election result loader

Also factor out logic to pick the loader for a particular mapping
because there are going to have to be lots of loaders.