Improve checks for migratable methods when throwing warnings

[spalladino]

In #164 we are checking if there are methods called initialize or migrate from the CLI, and showing a warning if they are not called when creating or upgrading a proxy.

A more accurate check if to test if the contract extends from Initializable or Migratable, and detect if any method has the isInitializer or isMigrate modifier. For the latter, we should also check whether the method needs to be called at all, or corresponds to a previous migration already invoked. It could also be the case that an upgrade operation introduces an initializer.

Note that this is subject to actually keep using Migratable. If we switch to standalone-contract-migrations, then this issue can be closed.