Allows users to supply default props to components

[NickyMeuleman]

It would be neat if users of the plugin could provide their own default props to be used in the components this plugin supplies.

Proof of concept here: https://github.com/NickyMeuleman/gatsby-mdx-embed/tree/default-props

Eg. I want tweets to be dark mode, but don't want to add theme="dark" to every tweet I add.
Instead I provide some default props via my gatsby-config

module.exports = {
  plugins: [
    {
      resolve: "@pauliescanlon/gatsby-mdx-embed",
      options: {
        defaultProps: { Tweet: { theme: "dark" } },
      },
    },
  ],
}

This will merge supplied props with the plugin's defaults.
So the resulting default props would be

theme: "dark"
align: "left"

since align:"left" is a default used by the theme and not overwitten in gatsby-config.

Would you be interested in this feature/me PRing my branch against this one and supplying more default props (branch currently only merges options for CodePen and Tweet)

[NickyMeuleman]

converted .js files to .ts (initially used method that used gatsby-node and thus didn't support TypeScript) and made a useDefaultProps hook that uses the props passed to the component and if they aren't there, falls back to the defaults (either provided by user or the default of the plugin)

TODO: TypeScript typing things

[PaulieScanlon]

@NickyMeuleman this is a great idea, did you mention it to me before, i'm sure someone did. Yeah please create a PR and we can discuss. Nice work!

[NickyMeuleman]

Yup. We talked briefly on Twitter where I floated this idea :)