Added support for macros 

The JSON settings file now supports "macros": This made more sense than my first attempt in https://github.com/thshdw/shuttle/commit/ae0f9896c4a151b740d153835409310b2384bd23 in which you had to add the string _macro_ in the title. Now everything under "macros" in the JSON file will be treated as a macro and run in the same terminal window.