FAKE build system integration

I wanted a step in my build process to automatically format all of the source files, to avoid coding style discussions in the future.

I realised that the helper functions I wrote would be better off upstream rather than in my `build.fsx` file. What do you think? Is it ok to have those functions in FantomasLib or should they be in a separate library for one reason or another?
The usual convention is `string list`, not `list<string>`. This applies for built-in generic types like `list`, `seq`, `option`, etc.