Add more information to pull request view

This adds if the PR is mergeable.  If it was merged it adds who merged
it, what time, and the merge commit.

[solomon@docker.com: rebased and resolved merge conflicts]
Signed-off-by: Solomon Hykes <solomon@docker.com>
Disable coloring if stdout is not a terminal

Signed-off-by: Solomon Hykes <solomon@docker.com>

This allows piping and redirecting the output of `pulls` without mixing
in term control characters garbage.