Add more basic commands

Hi,

I think that all the options that are available in Zazu icon context menu should be available as commands.

• It can be very useful to add an about command, that will open the Zazu about (and you can also add the os and git version, so it would be super easy to get those details for issues for example).
• Same for update plugins, check for updates and reload config

Zazu: 0.5.2 ; OS: win10 ; GIT: 2.12.2