Add FromXmlParser.Feature.PROCESS_XSI_NIL to allow disabling processing of xsi:nil attributes on reading

[cowtowncoder]

(note: sort of related to #467)

Although xsi:nil usually makes sense to be used, it probably makes sense to NOT process it when reading as JsonNode (just as an example). It seems simple enough to add a new feature in FromXmlParser.Feature (default: enabled) for something like

FromXmlParser.Feature.PROCESS_XSI_NIL

The main use case would be to allow preserving the attribute (and possible contents, if any): when processing is enabled it will be coerced into null which may not always be desired.

[cowtowncoder]

Added for 2.13.0-rc1.

One unfortunate limitation: will NOT work for root element in 2.x since feature flag override only occurs after parser initialization. Fixed in 3.0.