Switch to elm-codegen for code generation

[miniBill]

This is a draft waiting for the new version of elm-codegen to be published.
It's published now.

This fixes #5

[miniBill]

Ready for review

[miniBill]

Poke

[rektdeckard]

Sorry for delay here, got lost in the shuffle. A few notes:

1. From a fresh clone, this doesn't work. First, there's no src directory to begin with, so the generate script just crashes. A little cleanup there would be nice; I.E. should be idempotent and should run correctly from an initialized state.
2. elm-codegen and elm-format appear to now be required dependencies to build, and should be included in the package.json devDependencies.
3. Something appears to be wrong with the Elm dependencies for codegen, because I cannot run the generation script without compilation errors:

-- MODULE NOT FOUND ----------------------------------------------- Generate.elm

You are trying to import a `Gen.Svg` module:

20| import Gen.Svg as Svg
           ^^^^^^^

...and many more like this.

Please fix this up so that any person cloning this repo for the first time and build and run it properly, and if necessary add some documentation if there are required CLI tools that can't be included as package dependencies.

[miniBill]

Ok, I've changed yarn generate to automatically run yarn and elm-codegen install as needed!

[miniBill]

Just noticed some issues with compilation, fixing them

[miniBill]

Blocked by mdgriffith/elm-codegen#101