Allow adding custom columns to some reporting tables

### Reproduction Steps
As reported in https://github.com/gocodebox/lifterlms/issues/873 (bullet III)
try to use [this code](https://lifterlms.com/docs/adding-custom-columns-reporting-tables-export-files/) in order to add custom column e.g. to the Courses table.

### Expected Behavior
I would see the custom column added filled with the custom chosen value .

### Actual Behavior
You will see the custom column added BUT filled with the column title rather than the custom chose value: [example ](http://dl3.joxi.net/drive/2019/05/24/0028/2950/1842054/54/efab9563f3.jpg)(from #873):

### Additional info
As @inokesh suggests in that issue, this is due to the fact that we're not allowing the filtering of the `get_data()` table method's output.

According to @inokesh, tables affected by this issue are at least:
- llms.table.courses
- llms.table.memberships
- llms.table.quiz.attempts
- class-llms-table-assignments
- class-llms-table-assignments-submissions