Add more beat options

[jminjie]

Add settings to switch to other beats (in 4 as well as 6).
Add a setting to adjust tempo.

[jminjie]

Its notable that you can actually acheive a 6/8 feel on the default beat with a specific circle shape (granted you still only get 2 high hats). Maybe one fun option would be to keep the 4/4 beat mode but with extra high hats and challenge the user to make it into 6/8?