Add support for keyboard shortcuts

[AlexTatiyants]

There could be two types of shortcuts supported: global and editor-specific. For global ones, the following features should be accessible:

• Open/Close settings panel
• Create new post
• Go to post list
• Sync to WP
• Open/Close metadata panel
• Get WP tags
• Get WP categories

Editor-specific shortcuts could include:

• text manipulation (bold, underline, italicize, emphasize)
• headers (at least the first 4)

[yuvipanda]

Not sure about shortcuts for editor specific things, but definitely for the other ones. We should also try to use platform specific shortcuts, so different on OS X, Windows and Linux. Also not sure if we need Shortcuts for Get Tags and Cats. Access keys might be enough for those?

Also how are we going to make these discoverable?

[AlexTatiyants]

Discoverability is tricky. I actually like the way Office does it: hit the Alt key and it shows you specific shortcuts underneath each item. We could also do it with simple tool tips on hover over.