Add support for papers under submission

[dojoteef]

There have been numerous reported issues regarding running aclpubcheck for submissions under review:

• #43
• #49
• #52
• #57

Reviewers are asked to ensure the papers they are reviewing meet the ACL formatting guidelines as part of pre-review checks:

Is the paper properly formatted according to the template?

Furthermore, papers can be desk rejected for violating these formatting guidelines. For these reasons, I think it's important to have some support for papers under submission. This PR does exactly that. If the --draft flag is passed on the command line, then line numbers in the margins are ignored, and one page is subtracted from the maximum allowed length.