Add global form option to disable tv4 validation

[maff]

Useful in cases where you want to set validation messages only manually (e.g. from an API response) and use angular-schema-form primarily to render forms. When this config option is set on in sf-options it will completely omit tv4 validation for the form. Example:

<form sf-schema="schema" sf-form="form" sf-model="model" sf-options="sf-options="{ tv4Validation: false }"></form>

[davidlgj]

Cool, this is a use case that never crossed my mind :) Do you still have a need for a schema? Would it be a good thing to be able to use it without a JSON Schema?

[maff]

For my use case I'm fine with JSON schema as I can generate all needed information on the backend.