Add catchAsync for Eithers

[otto-dev]

Closes #105. Allows to use catchAsync in place of catching for asynchronous contexts.

Example usage:

  Future<Either<dynamic, FooBar>> doAsyncThings() {
    return catchAsync(() async {
      final foo = await asyncAction();
      final bar = await asyncAction();
      return foo + bar;
    });
  }

Went for catchAsync instead of catchingAsync.