Add configuration for disabling external development

[t-kelly]

Problem

Slate Beta 7 reintroduced Browsersync to Slate, which required Slate to switch from references files on https://localhost:3001 to https://xxx.xxx.xxx.xxx:3001 (local IP) in order to access the local theme from external devices on the same network.

It would be nice to be able to disable external testing using with local IP and use localhost whenever you please. Lets add a config for that.

[lock]

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