Netstandard 2.0, redo build.fsx, version updates

[PaigeM89]

Updates a whole bunch of dependencies & tools.

I used the build.fsx script from Miniscaffold and completely overwrote what was in place. There was a lot of tooling involved in the existing script that all interacted in specific ways, and instead the new version just calls yarn to invoke scripts in webpack.config.js, which probably better mirrors common use cases.

I removed the final docs tooling targets & target dependencies because I wasn't able to get the docs to generate in some test projects - lots of things seem out of date, lots of errors relating to FSharp.Literate not being able to parse scripts, etc. I think it's better to tackle this later.

Fixes #23 and #25 (by upgrading straight to Fable 3)

[TheAngryByrd]

Probably worth moving over to github workflows as well instead of using Travis/Appveyor