Instead of doing return, insert an `if` block to skip emitting zero fields.

This also allows the check to be inserted at any level.

Fixes #376

Emitted code:

```Go
// MarshalMsg implements msgp.Marshaler
func (z TypeSamples) MarshalMsg(b []byte) (o []byte, err error) {
	o = msgp.Require(b, z.Msgsize())
	o = msgp.AppendArrayHeader(o, uint32(len(z)))
	for zb0004 := range z {
		// check for omitted fields
		zb0001Len := uint32(2)
		var zb0001Mask uint8 /* 2 bits */
		_ = zb0001Mask
		if z[zb0004].K == 0 {
			zb0001Len--
			zb0001Mask |= 0x1
		}
		if z[zb0004].V == 0 {
			zb0001Len--
			zb0001Mask |= 0x2
		}
		// variable map header, size zb0001Len
		o = append(o, 0x80|uint8(zb0001Len))

		// skip if no fields are to be emitted
		if zb0001Len != 0 {
			if (zb0001Mask & 0x1) == 0 { // if not omitted
				// string "k"
				o = append(o, 0xa1, 0x6b)
				o = msgp.AppendUint32(o, z[zb0004].K)
			}
			if (zb0001Mask & 0x2) == 0 { // if not omitted
				// string "v"
				o = append(o, 0xa1, 0x76)
				o = msgp.AppendUint32(o, z[zb0004].V)
			}
		}
	}
	return
}
```

If only 1 field, the check is omitted (and there is similar behavior on clearomitted).

Fixes #266

This adds `msgp:newtime` file directive that will encode all time fields using the -1 extension as defined in the [(revised) messagepack spec](https://github.com/msgpack/msgpack/blob/master/spec.md#timestamp-extension-type)

ReadTime/ReadTimeBytes will now support both types natively, and will accept either as input.

Extensions should remain unaffected.

Fixes #300