Update keyboard shortcuts for switching tabs

[PalmerAL]

• Change cmd+n to switch to the tab at index n, rather than going n tabs to the right (fixes Using ⌘+number to switch tabs #38). The original idea behind this was to make it easier to navigate when there are more than 10 tabs open, but all of the feedback I've heard on it is negative. It also doesn't work very well on macOS because some of the shift+cmd+n shortcuts are also used for screenshots. With this PR, we now match what other browsers do and use absolute indices.
• Added shift+mod+[ to go to the previous tab and shift+mod+] to go to the next - this works in other browsers but was missing from Min.