Added keyboard shortcuts to enhance editing

[therealtakeshi]

I really like frame.js, and I've made a few things with it, but there have been some annoyances as I've been using it for another project. So I thought I'd take some time to add some functionality to really help me use the tool more+better. I'm happy to take critique and make changes if needed for accepting.

The code is fairly simple thanks to the Mousetrap keyboard event library, but basically:

• Holding Shift scales down the amount of change while traversing timeline with arrow keys and while dragging blocks
• Holding Cmd/Ctrl scales up that same amount of change
• Holding Alt snaps the new values (timeline value, or event start/end) to larger increments so you don't have to be so precise while dragging
• Tapping Shift+D or D twice with a block selected will duplicate it as if you'd clicked the menu option
• Tapping X or Delete with a block selected will remove it as if you'd clicked the menu option

Also updated the README to clarify the key commands, but I get it if you'd prefer keeping it real slim.