encoding/json: add Encoder.DisableHTMLEscaping

This provides a way to disable the escaping of <, >, and & in JSON
strings.

Fixes #14749.

Change-Id: I1afeb0244455fc8b06c6cce920444532f229555b
Reviewed-on: https://go-review.googlesource.com/21796
Run-TryBot: Caleb Spare <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: cespare