Add support for XMLConstants.FEATURE_SECURE_PROCESSING via SAX/Stax factories

[cowtowncoder]

note: behavior clarified in JEP=185). Also related: #51

So: to support more standard JAXP/Staax configuration, let's add support for "secure processing" configurability. This will be in addition to Woodstox-specific (and more convenient, but only if using Woodstox) configuration options... although looks like there was no "configureForSecurity()` yet, so we might as well add that as an alias.

[cowtowncoder]

Added skeletal support for both Stax and SAX factories: enabling via setProperty/setFeature (Stax / SAX) will disable resolution of external parsed entities, which seems like the only obvious setting.
Since Woodstox already sets reasonably size limits, did not add stricter settings although if someone suggests better settings I am open to further changes.

Note that I did not add support for either system property or jaxp.properties -- if those are desired, please file a separate issue.

[cowtowncoder]

Will be included in 5.3.0 to be released today.

[deanshapira]

@cowtowncoder is there a CVE associated with this security vulnerability?

[cowtowncoder]

@deanshapira Not that I know of. Since it simply bundles enabling of existing settings I don't think it would qualify.