Feature: view your pulls

[adunkman]

It’d be sweet to see a list of my open pulls, most recent on top, so I can jump quickly to a pull.

gh my pulls seems like a natural place, but it already has another behavior.

@gharlan What do you think? I can take a stab at it, if you like it.

[pjvandehaar]

Now that it's implemented, can you close this PR?