Agile Async methods

I propose a new modifier for Async methods `Agile` which would basically insert `.ConfigureAwait(False)` after every _awaited_ expression automagically. We'd probably not do that if the expression were a ConfigureAwait call to avoid redundancy. It sucks that we make people insert this call at the site of EVERY Await. We could solve this with a modifier and maybe even an assembly wide option for class libraries. I'm sure there are several million lines of Roslyn code we could clean up with this.