Default props

[NickyMeuleman]

ref #13

Allows users to specify default props in gatsby-config.
These will be combined with the plugins default props.

Components will use the props passed to them in .mdx
If no props are specified in .mdx, the defaults in gatsby-config will be used.
If no props are specified in gatsby-config, the defaults set be the plugin will be used.

How

By adding a React context around the app in provider.
Individual components then access that context and merge the received props with the default ones (used via a custom React hook, useDefaultProps )

[NickyMeuleman]

Added defaultprops to components that use them.
Typescript is showing errors because the returned type from the useDefaultProps hooks doesn't include the props that were not specified, but have a default.

Do you know how to accomplish that @PaulieScanlon?

edit: also the @pauliescanlon/gatsby-mdx-embed/hooks/useDefaultProps.js file is tracked, which is a generated file. Will have to remove that one from git tracking.
edit2: Removed some redudant typings of React Props.
Since React.FunctionComponent<IProps> is being used. The props do not need to be typed seperately, they already are by that <IProps> generic that is passed in.

FunctionComponent<IProps> typing definition below:

    interface FunctionComponent<P = {}> {
        (props: PropsWithChildren<P>, context?: any): ReactElement | null;
        propTypes?: WeakValidationMap<P>;
        contextTypes?: ValidationMap<any>;
        defaultProps?: Partial<P>;
        displayName?: string;
    }