support additional on-demand "custom" fields

[lsloan]

The LTI standard allows any field that begins with "custom_". For example, see the Caliper ones. Some LTI providers may expect other custom fields specific to them. Since supporting each other provider in littLTI currently requires modifying the HTML or JavaScript code, add a feature to allow the user to enter other fields without coding.

The best way to do this would be to let the user click a button that adds a "custom_" field. The user would be able to edit the name and the value of the field.

Such a feature would be especially powerful if littLTI also supported saving parameters. (Another issue for such a feature will be opened.)

[lsloan]

This issue was moved to tl-its-umich-edu#1