Fix usage for list of strings and add some bang functions

[jesse-iluminai]

Originally provided by manuel-rubio from gjaldon#110

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The following example in README.md was incorrect:

use EctoEnum, "one", "two", "three"

But even the correct one was not working:

use EctoEnum, ["one", "two", "three"]

I moved the check from defenum to be in use for EctoEnum.Use module and now it's working correctly.

In the same way I missed a couple of functions I usually use like cast!/1 and dump!/1. Both were added as well.