Add DieWith method (#106)

DieWith passes the current die to a callback function as a mutable die.
This is usefull to apply predefined functions that operate on the die
while preserving the fluent nature of the api.

Signed-off-by: Scott Andrews <[email protected]>

README.md

@@ -163,6 +163,10 @@ type MyResourceDie interface {
  // either mutable (`false`) or immutable (`true`). 
  DieImmutable(immutable bool) *MyResourceDie
 
+ // DieWith returns a new die after passing the current die to the callback
+ // function. The passed die is mutable.
+ DieWith(fn func(d *MyResourceDie)) *MyResourceDie
+
  // DeepCopy returns a new die with equivalent state. Useful for
  // snapshotting a mutable die.
  DeepCopy() *MyResourceDie