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Proposal from the #7 comment thread.

- It was a bit complex to get things right without revert to 1.7.0, what I've done before new clear commits. Sorry for the mainline.

- Since you "deepFreeze" the config object, we can't easily use the (nice) syntax from brunch/src/helpers.coffee @es128 suggest. I kept the default hash and added the `config.plugin?` check from https://github.com/brunch/auto-reload-brunch/blob/master/src/index.coffee#L11:L13

- Coffee, JS & README are synced on each commits.

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