Add `MapperFeature.ALLOW_COERCION_OF_SCALARS` for enabling/disabling coercions

[cowtowncoder]

(see #200 for background; #1095 also)

While existing features should cover cases of allowing (or not) of coercion from null to primitive (FAIL_ON_NULL_FOR_PRIMITIVES), as well as coercion from empty String to primitive via null (as per #1095), there is no way to block other coercions from JSON Strings (primarily) or, in case of boolean/Boolean, from integral numbers.

To allow preventing such coercions, it would make sense to add yet another DeserializationFeature, named something like ALLOW_COERCION_FOR_SCALARS, defaulting to true to keep current behavior as the baseline.