Add option to hide solution before submitting

[adamblake]

Summary

Adds a setting to remove the solution from view before submitting an answer. Accomplished by adding a data-show-solution-before = "false" setting to the setup and results in hiding the solution button until after the Submit button has been pressed.

Details

In our use of the DataCamp modules we would like students to first attempt to make an answer before being shown the solution we are grading them against. See #100 for a very similar use case where another of our team members requested to hide the solution altogether.

With this I essentially followed the patterns with the show-run-button option for adding the option, and then added a boolean state variable to flag when the item has been submitted.

[adamblake]

Did not mean to close -- sorry!

[hunkim]

I think this is a very good feature.