Add git alias for checking out a PR on GitHub?

[salcode]

Previously (in version 1.x.x) we had this functionality as part of the bash aliases (which have since all been removed (see #120)).

# gpr 96, will fetch PR 96 create branch pull-request-96
gpr () { git fetch origin pull/$1/head:pull-request-$1; }

Do we want to add a git alias for this instead?

e.g. git ghpr 96