Add grade/comment functions for submissions

[stillmatic]

Adds grade_submission() and comment_submission() to assign grades and comments to users' submissions to a given assignment.

In my use case, we split up grading for exams, and coordinate via a shared Google sheet. Instead of giving comments twice, we can instead make our grades and comments once in the shared spreadsheet and then load them into Canvas using these functions. We have some table like this:

user_id	grade	comment
101	75	Missing Q3
102	100	Great job!
103	50	See me.

These new functions make it easy to add these grades, especially using purrr::map2(), etc.

A few other misc changes, mostly whitespace.

[daranzolin]

Awesome Chris. I'd gladly accept a PR that acknowledges your contributor role in the package DESCRIPTION too.