make_submission: Make a magrader submission object
In garrettgman/magrader: A simple checking scheme for tutor based on magrittr pipes

A magrader submission object contains six parts: 1. ‘answer' - The student’s original answer (stored as a string) 2. ‘focus' - An optional portion of the student’s code parsed as an expression tree 3. 'status' - The current status (pass, fail, warn) 4. 'warnings' - A set of warnings 5. 'praise' - A set of praise messages 6. 'failure' - A set of failure messages

1 2	make_submission(answer, focus = NULL, status = "pass", warnings = NULL, praise = NULL, failure = NULL)

`answer`	- The complete, original student submission as a character string
`focus`	- A portion of the student answer parsed as an expression tree
`status`	- The current status of the submission: "pass", "fail", or "warn"
`warnings`	- Warning messages as a vector of character strings, to be displayed if final status is warn
`praise`	- Praise messages as a vector of character strings, to be displayed if final status is warn or pass
`failure`	- Failure messages as a character strings, to be displayed if status switches to fail