Allow selectively showing more context in diffs

[tarsius]

It is already possible to show more context when needed. For example the --function-context argument can be added on the fly using D - f g and the number of context lines can be increases using +.

But sometimes one wants to only show more context for a single hunk. Github at least does provide that feature, before every hunk shown there, there is a "expand" button, which shows more context before the hunk.

Magit should also gain this minimal-viable implementation, and then go from there.