Integrate FSharpLint into build

The [FSharpLint](http://fsprojects.github.io/FSharpLint/) tool can check F# code for common errors/improvements and can make suggestions on how to fix them. We should include this in our build, so that it runs each time a project runs.

Note: this is dependent on having a separate project for each exercise, which we will do once the .NET Core tooling is fully working.