Improve indication of "source reference" in JsonLocation wrt byte[],char[]

[cowtowncoder]

(note: inspired by #343)

Since we already include contents when read from String, it seems reasonable to do about the same for byte[] (assume UTF-8) and char[] sources.
But it'd also make sense to add a JsonParser.Feature to allow disabling inclusion of content, for security reasons.

[cowtowncoder]

Added JsonParser.Feature.INCLUDE_SOURCE_IN_LOCATION (default: true), disabling of which will prevent addition of source reference, usually for security reasons.

Also: maximum content snippet displayed limited to 500 characters.