Enhancement: Add `finalize` effect, triggered on unmount (symmetry with `initialize`)

[agurtovoy]

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• 2018-08-21T23:33:31.882Z:b85b485: The pull request was created
• 2018-08-21T23:35:44.458Z:b85b485: @agurtovoy checked I have read and will comply with the [contribution guidelines](https://github.com/FormidableLabs/freactal/blob/master/CONTRIBUTE.md)..
• 2018-08-21T23:35:48.316Z:b85b485: @agurtovoy checked All related documentation has been updated to reflect the changes made. _(required)_.
• 2018-08-21T23:36:22.003Z:b85b485: @agurtovoy checked I have read and will comply with the [code of conduct](https://github.com/FormidableLabs/freactal/blob/master/CONTRIBUTE.md)..
• 2018-08-21T23:43:59.494Z:b85b485: @agurtovoy selected minor as the semantic version.
• 2018-08-21T23:44:02.777Z:b85b485: @agurtovoy checked My commit messages are cleaned up and ready to merge. _(required)_.

[agurtovoy]

Not sure why maintainerd is complaining about cc4ba78; it does have an empty second line. Maybe an old version of maintainerd (https://github.com/divmain/maintainerd/issues/1?)