Use fetch instead of superagent

[LaurensRietveld]

We use superagent for all http requests. This is a left-over from the old yasgui version. Let's use the native fetch api instead. This should solve #144 as well, as fetch doesn't do any 'magic' response body parsing

[ashleysommer]

Any progress on this yet? I've started using Yasqe in a new JS project, and I'm using fetch for all other HTTP operations in my code, I'd prefer if Yasqe could use fetch natively too.

[lucas42]

For anyone still watching this, looks like it's been addressed in a fork of the library: https://github.com/zazuko/Yasgui/pull/19