Add set() and revert() to Initial plugin

[from-nibly]

Implements #208

This allows you to reset what "initial" is for things like editing forms when you save the form but might want to "edit" and "cancel" after saving.

Since set(s) takes a state value you can use it to set it to any arbitrary state.

The revert() method sets the current state to whatever initial was. This could technically be accomplished with Initial(state).set(Initial(state).get()), but would be a handy shortcut.

[codecov-commenter]

Codecov Report

Merging #209 (7807a16) into master (2ff4d5f) will not change coverage. The diff coverage is n/a.

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  Files           1        1           
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  Branches      170      170           
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  Hits          536      536           
  Misses         67       67           

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[from-nibly]

@avkonst made some updates. I found some other issues with the set and revert functions and those are also now fixed.

Unfortunately I'm not sure I have time right at the moment to really test the nested state stuff. It is working for flat state at the moment.

I can probably get around to properly testing it in a few days or so if you want me to make sure that's working before merging it.