[Feature] Remapping gauge

[vid8687]

Add support for remapping needle numbers to chosen gauges (full and half, possibly other a/b gauges)

[ixchow]

Possible interface:

let k = new knitout.Writer();
//right now, k.mapper === null, which implies full gauge
k.knit('+', 'f' + 7, '6'); //knits on needle f7
k.setMapper(new knitout.HalfGaugeMapper());
k.knit('+', 'f' + 7, '6'); //knits on needle f17
k.unmappedKnit('+', 'f' + 7, '6'); //knits on needle f7

Where a Mapper is an object with two functions:

function MyMapper() { }
MyMapper.prototype.toMachine(bed, needle) {
    return {bed:bed, needle:needle*2};
}
//We could probably eliminate this one, but it turns out to be useful in certain cases:
MyMapper.prototype.fromMachine(bed, needle) {
    return {bed:bed, needle:needle/2};
}

This is not the only possible interface. For instance, a 'Mapper' could be an object that takes a knitout.Writer and proxies calls to it:

let k = new knitout.Writer();
let k2 = new knitout.HalfGaugeMapper({writer:k});
k.knit('+', 'f' + 7, '6'); //knits on needle f7
k2.knit('+', 'f' + 7, '6'); //knits on needle f17

Open questions:

• how do we deal with mapping in the context of querying active needles on the machine? (I.e. if mapping is enabled, do we report actual machine locations that are occupied, or the needles that map to them?)
• how do we handle switching gauges?
• how do we support user mappers?