type definition for ParserBuildOptions does not allow plugin options

[cyberwombat]

The ParserBuildOptions type does not allow passing plugin options such as :

const parser = generate("start = ('a' / 'b')+", {
  output: 'source',
  format: 'es',
  plugins: [tspegjs],
  tspegjs: {   // <-- not allowed
    customHeader: 'foo'
  }
}) 

[hildjj]

I wonder if we can just add this to ParserBuildOptions: [extensionOpts: string]: any;