allow custom fields in the custom report method

[tangerilli]

See the note about custom fields at the end of the field list for details

[internetlee]

@tangerilli - thank you for looking into this. i flagged this #29, too, and came back to see if i could tackle a solution. this looks great! i think there may be another piece or two to consider here.

if not f.startswith('custom') and not self.employee_fields.get(f):

only non-tabular fields are assigned prefix 'custom'. custom tables are assigned a four character int formatted as a string [a custom table we have is 430{x} where x+=1 , or a float formatted as a string - searching for this example again. cannot recall which field we have for this - when i find it i'll show the example.

thanks again!