Passing any Path to MSBuild, not just FilePath

It's fairly rare for me to want to specify a file path for MSBuild. Usually the path which shows most clearly what I really care about is either `"."` or `"ProjectDir"`.

```cs
var someProjectDir = Directory("SomeProject");

// ...

MSBuild(someProjectDir, …);
```

The error this causes:

> error CS1503: Argument 1: cannot convert from 'Cake.Common.IO.Paths.ConvertableDirectoryPath' to 'Cake.Core.IO.FilePath'

If we change the first parameter of the MSBuild alias to `Path` instead of `FilePath`, would passing a `ConvertableDirectoryPath` just work?
Would you accept a PR for this? 😃