Add IEnumerable<T> support for arguments

Would be nice if we supported passing multiple arguments to the Cake.exe.

cake.exe --foo=1 --foo=2 --foo=3

And allowed the user to resolve them all by resolving an IEnumerable<T> like this:

var foo = Argument<IEnumerable<int>>("foo", Enumerable.Empty<int>());

Of course there would have to be safe guards in place to ensure that we throw if the user try to resolve a single argument where multiple has been passed and vice versa.

(Originally reported at http://stackoverflow.com/questions/35727010/how-do-i-pass-an-argument-that-is-a-string-array-to-my-cake-script)

[gitfool]

I also thought that commandline was a good option if Cake was to improve it’s command-line support. It’s a bit confusing and inconsistent having different parameter passing done by PowerShell / Bash (in the bootstrapper) and then by Cake. It would be better if Cake did everything and the bootstrapper did nothing but bootstrap.

[fleed]

Any update on this one?