Duplicate column names support

This makes it easier to re-use stored procedures that have duplicate
column names for multi-mapping between Dapper and tedious. When the same
column is used multiple times, you can reference it with:
row['columnName'][index].value

Single occurence columns would still work with row['columnName'].value.
There might be a more intuitive way of references the duplicated column,
so I'm open to ideas there.