Allow generated file path to be configurable

[airtonix]

fixes #32 make destination configurable.

be warned, this is a really rough first pass at this.

// next.config.js

import withTypeSafeRoutes from 'next-type-safe-routes/plugin';

withTypeSafeRoutes({
  nextTypeSafeRoutes: {
    destination: '__generated__/next-type-safe-routes/',
  } 
});

[osdiab]

i wanted to push to your branch but i didn't have privileges so just opened up my fixes in another branch.