a8n: add branch flag for creating campaigns - #96

Idea as suggested by @sqs in https://github.com/sourcegraph/sourcegraph/issues/7850#issuecomment-577674931:

It would be cool if the branch name could also be specified in the Markdown file by using a comment, such as:

Title of the campaign

Description

// branch = my-branch-name

There's precedent here: sometimes blog engines or static site generators allow you to add "front matter" to a page, in the form of YAML/TOML/JSON (see Hugo's explanation of it here: https://gohugo.io/content-management/front-matter/)

I don't think we need to fully support that, but some way to declare a line as "meta" would be great.

Update on my previous comment here: it's more important to get this merged with the basic functionality (-branch flag!) than the additonal functionality of specifying the branch name in the markdown file.

Merging this now since https://github.com/sourcegraph/sourcegraph/pull/8111 has been merged.

I also added a commit that documents this new flag and prints it in the friendly output message.