Group qualifier not implemented

[mbklein]

Level 2 Group Qualification isn't working correctly. When the qualifier comes before an element, it applies only to that element. When the qualifier comes after the element, it applies to all elements to the left.

Expected:

iex(1)> EDTF.parse("1978?-02-23/1978?-02-24")
{:ok,
 %EDTF.Interval{
   start: %EDTF.Date{
     type: :date,
     values: [1978, 1, 23],
     level: 0,
     attributes: [uncertain: 15]
   },
   end: %EDTF.Date{
     type: :date,
     values: [1978, 1, 24],
     level: 0,
     attributes: [uncertain: 15]
   },
   level: 2
 }}

Actual:

iex(1)> EDTF.parse("1978?-02-23/1978?-02-24")
{:error, :invalid_format}

[mbklein]

Fixed by #14