Cleanup RazorEngine files.

[matthid]

Some background: Antaris/RazorEngine#244

Basically RazorEngine was leaking temp files sine 3.5.0. Now I have a fix/workaround in place but it only works when RazorEngine is not used in the default AppDomain. So we need to take additional actions here in F# Formatting (because the primary usage is via FSI, which sadly uses the default AppDomain..).

Because spinning up AppDomains in F# scripts will be very difficult (to get a dynamic assembly from one into another AppDomain), I can only see two options:

• Spin up the AppDomain internally in F# Formatting (if we are in the default AppDomain)
  But I guess this is only possible for high level APIs like MetadataFormat.Generate (I don't think we can support high order functions)
• Handle this in FAKE: Spin up a new AppDomain within FAKE and run the build-script from there
  Of course we need to also change https://github.com/fsprojects/ProjectScaffold/blob/master/build.template#L199 and spin up another FAKE process instead of calling fsi.exe.
  At this point everything should work fine.

IMHO we should opt for 2 (it should be easier to implement as well) and leave F# Formatting unchanged, however I would like to hear what you think.
/cc @tpetricek @forki