Add POINT(X, Y) support

[kevinmartin]

Adds POINT(x, y) support. Assumes an object with constructor name 'Point'.

Can be easily added like so:

// Simple:
function Point(x, y) {
    this.x = x;
    this.y = y;
}

// Verbose (ES6):
class Point {
    constructor(x, y) {
        this.x = x;
        this.y = y;
    }
}

// Escaping:
SqlString.escape(new Point(123, 456)); // -> POINT(123, 456)

I wrote it this way since, judging from the conversation in mysqljs/mysql#877, almost anything could break SqlString.objectToValues(...).

[kevinmartin]

Maybe instead of a BYO Point Class, it can be included in the library as SqlString.Point(x, y)?

[dougwilson]

Yea, this is a much better implementation than the other pull request. I'm wondering if we may actually be better served by looking for a .toSQL method on an object and calling that (or .toMySQL() ? .toSQL('mysql')?) and letting the object generate whatever SQL it feels is most appropriate.