UX improvements and support .yml files

• We tell people Gatsby sites work w/o Javascript and then people fire up a development server w/o javascript... and see a blank screen. The development server actually does need javascript to power hot-reloading, etc. @nason add a default <noscript /> message for people who've turned off Javascript to tell them what's happening and how to test their site w/ no javascript (gatsby build; gatsby serve-build) #566
• @vinnymac borrowed a nice recent addition to create-react-app where they're not just telling you that your desired port is taken but also which process is (probably) using it #579
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• @briancappello added support for the .yml file extension (a less common variant file extension for YAML files). #580

Thanks everyone!