Use the Maybe type when building a new Comment from json for commit_id and path.

I ran into a JsonError when trying to create a comment on an issue since Comment's commit_id is an optional value, so I am marking it and deserialize it as such. Same thing for Comment's path.
The code that was failing was:
```haskell
import Github.Auth
import Github.Issues.Comments
let auth = GithubOAuth "token"
createComment auth "repoowner" "reponame" 34 "Adding comment from ghci"
```