Enforce option value via types

[Tyriar]

I think we can extend StringOption and similar to actually check the value at compile time, not just the type.

https://github.com/sourcelair/xterm.js/blob/39328d6e0c05b1f1f574303ad0288687ca6f813f/src/Types.ts#L33-L35

[Tyriar]

I think this is already done in setOption:

    setOption(key: 'bellStyle', value: null | 'none' | 'visual' | 'sound' | 'both'): void;
    /**
     * Sets an option on the terminal.
     * @param key The option key.
     * @param value The option value.
     */
    setOption(key: 'cursorStyle', value: null | 'block' | 'underline' | 'bar'): void;