Workflow improvements, window stability

• Window will re-open after it has been closed (even if it was closed using the red X button).

• Window will refresh on `cmd-ctrl-h` if it is already open.

• Window now has 'Refresh' button.

• If current document changed, the Window will refresh to reflect the JSHint results from the new current document.

• Mixin is now scoped to JS documents, if user runs mixin against a non-javascript document there'll be an alert (instead of an empty window with a meaningless number of errors in the title).

• If the window does not have focus while user runs mixin using `cmd-ctrl-h` to zero errors result, the No-Errors alert will be shown and the window closed.

• Will fail silently if there is no open document.