Add operator “overloading”

[cshaa]

The options object should allow for methods like customAdd and customGreaterThan that would override the default action of these operators.

[cshaa]

In the actual implementation, I went with this API:

interface Options {
  // ...
  
  operators: {
    '+': (a, b) => any,
    '-': (a, b) => any,
    // ...
  }
}

The minus operator overload is used for both the binary and the unary operator:

• -a will result in operators['-'](a)
• a - b will result in operators['-'](a, b).

The modulo operator can be overloaded using operators['mod'].