Add a toggle to disable "omitempty"

[ptodev]

Adding a toggle to not include the "omitempty" tag from generated struct fields.

This could be useful if you're migrating an already existing codebase which doesn't use "omitempty" to go-jsonschema and want to minimise the number of changes.

The codebase which I'm working on uses mapstructure and almost never uses omitempty. Instead, it has decoder hooks to manipulate the decoding in various ways. To be honest, I think having the omitempty might not change anything about the decoding process in my particular case. But I think having such a toggle could help make sure there are no unintended consequences from having an omitempty.

I don't think most people would want to have a DisableOmitempty = true though. If you use json or yaml decoding, the validation functions which go-jsonschema generates wouldn't work correctly if omitempty is missing, since the validator wouldn't know that a non-required property is missing and it wouldn't be able to set it to its default value. My change is probably only useful for certain users of mapstructure.

An alternative way to do this change could be to add it to goJSONSchema setting so that we can override it per property, instead of setting it globally. Maybe we should have both types of toggles - a global, and a property-specific one?