drop scalajs support

[pepegar]

The way the library is built, there are two big stoppers to keep supporting scalajs:

• nodefetch
• contextual

I myself don't use scalajs, and I think the library doesn't have a lot of users. If anyone wants to keep supporting scalajs, I'm more than happy to guide and accept new contributors.

[antonkulaga]

@pepegar one of the reasons I used your library was cross-building to scala.js, I probably have to migrate now

[pepegar]

The real problematic part is node. Do you use the library in browser apps or in node apps?