optionally make refresh hook into magit-refresh

[wbolster]

magithub is wonderful.

but.

i really do not like how it does network requests everytime i (magit-refresh) the magit status window, which i do quite a lot, since the github requests are slowing things down significantly.

it would be great if magithub:

• had a defcustom (defaulting to t to keep the current behaviour) to disable the automatic sync in magit-refresh
• offered an explicit magithub-refresh command, which would also be available from the magithub submenu in the magit popup window.

[wbolster]

closing since this should've been reported against another repo with the same name.

https://github.com/vermiculus/magithub/issues/27