Add function prop for labelling keys. Also changed some props for clearer usage.

Orientation is limited to 90-degree intervals.

Support MIDI event handling by suppling a `MIDIInput` object through the `midiInput` prop.

Make component single channel only, in order to have a more concise API.

Keyboard map is the one responsible for handling mouse, touch, and keyboard events. This is to keep the keyboard rendering and external events handling separate.

Add SSR support by checking to render server-side or client-side.

Also made key offsets as trade-off of accuracy and pixel-perfectness.

Adding width to calculation does not affect the `getKeyWidth` and `getKeyLeft` methods.

This is to cover repainting of keys using the widths.

Prefer using React memoization instead of mem when using inside React.

Add memoization to expensive pure functions because the library involves a lot of computation under the hood.

Make custom theme scalable and more flexible in terms of customization.

Use custom styled keys and add pressed keys for styled key set.