Support for generating SVG components without `.defaultProps`

[Grohden]

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This should close https://github.com/airbnb/babel-plugin-inline-react-svg/issues/126

I've changed the code to:

• Use inline 'defaultProps' in the generated SVG components
• Use object.assign to merge the prop overrides (see linked issue)
• Use an option (emitDeprecatedDefaultProps) that allows this package users to still keep emitting the defaultProp declarations

I've made the option to omit default props an opt-out, so it is a breaking change.. but up for the author to change that to a default false