Add Github Pull Request entry in magit-dispatch-popup.

[PierreTechoueyres]

This patch permit to add an entry in magit-dispatch-popup see bellow :

Popup and dwim commands
 A Cherry-picking          b Branching               B Bisecting               c Committing
 d Diffing                 D Change diffs            e Ediff dwimming          E Ediffing
 f Fetching                F Pulling                 l Logging                 L Change logs
 m Merging                 M Remoting                o Submodules              O Subtrees
 P Pushing                 r Rebasing                t Tagging                 T Notes
 V Reverting               w Apply patches           W Format patches          X Resetting
 y Show Refs               z Stashing                ! Running                 # Github PR

[jb55]

👍 lgtm

[alexander-yakushev]

Thank you!