Make the flow/prop types for Synth and Filter type more granular

[fson]

react-music is awesome 💥

I found myself looking at the flow types of props to see what values I can use, so I added typings for the type prop of Synth and Filter to make them more self-documenting.

I searched the Web Audio API spec for enum, and I think all enums currently used from there are now typed 😄

[kenwheeler]

Thanks dude!