Added {useDefault: true} option flag to substituted in default values

[jonasfj]

Fixing issues #48.

Basically the useDefault option flag will allow substitution of default values from JSON schema.
These will nest and they will not work for required properties. Validation will also validate the default value. Ideally we could do this at compile-time.

It works by mutating the input, so it only works if top-level value is an object or array. But this seems like a reasonable limitation. It's very very nice that we get default substituted in along with validation.
Proper warnings about this being a non-standard feature and it being mutating the input object have been added to README.md.

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I generally agree that features like useDefault and filter are dangerous. And they should definitely be hidden behind feature flags. But hiding them in methods like validator.filter seems to make little sense, one of the primary features is this being fast and getting the validation done along with default value substitution. + the ability to combine filter and useDefault.

Note, I understand anyone having mixed feelings about adding non-standard features like this. But in practice they are very useful and they have no performance overhead as they are enabled/disabled at compile-time.

[mafintosh]

could you remove the log?

[jonasfj]

Sorry about that console.log... Pretty sad that I managed to somehow slip that into a 6 line patch :)
After all everything else is just tests and docs.

[calebmer]

Could this please be merged?

[RangerMauve]

This'd be nice

[jonasfj]

Note, this PR has some limitation with respect to anyOf, etc, see discussion here about the complexities of doing this right: ajv-validator/ajv#42 (comment)