Implementation for this story https://trello.com/c/gbBtpARq/709-1-define-default-html-file-for-live-development

https://trello.com/c/gbBtpARq/709-1-define-default-html-file-for-live-development

Implemented fix for https://github.com/adobe/brackets/issues/2033

Added a lot of test cases to cover the new functionality.