Optional command to add a file header in each generated file

[crhaglun]

Add an optional parameter --fileHeader to set a header at the top of each generated file.

This is a legal requirement from my company, any generated code that we use needs to contain information about what tool generated the code.

With this change, a command line like

.\FFmpeg.AutoGen.CppSharpUnsafeGenerator.exe --fileHeader "/// <auto-generated>
///     Generated by FFmpeg.Autogen; https://github.com/Ruslan-B/FFmpeg.AutoGen
/// </auto-generated>"

Will place this comment at the top of each generated file:

/// <auto-generated>
///     Generated by FFmpeg.Autogen; https://github.com/Ruslan-B/FFmpeg.AutoGen
/// </auto-generated>