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?
Beware that `formatSourceString` throws `FormatException` if there is parsing error. Would you like to handle it?

This is how I usually do https://github.com/dungpa/fantomas/blob/3373233d50be7d00b02134ff4b779b0a86db92b3/src/Fantomas.UI/CodeFormattingCommands.fs#L72-L92.

You shouldn't trust output results (there are bugs), `isValidFSharpCode` is used for double check.