Replace node-fetch with slim-fetch

[Kwuang Tang]

Is your feature request related to a problem? Please describe.
node-fetch is larger size than it should be, slim-fetch is much lighter with identical implementation and API.

Describe the solution you'd like
Replace node-fetch with slim-fetch.
https://www.npmjs.com/package/slim-fetch is a new library that has basically identical code to node-fetch, but stripped away unnecessary parts of the distribution (eg. node-fetch actually has three copies of the same code for backwards compatibility purposes).

Describe alternatives you've considered

Additional context