Make the output dir of the map files can be modified

[hoah2333]

For now, you can choose to save the css files into another directory, but the map files will always be saved into the same directory with the css files. I want to add a setting to modify the map saving directory, so that you can put the css files in ~/css and put the map files in ~/map, which can make the directory tree much neater.

[glenn2223]

Sorry for the delay and thanks or the feature suggestion.

I don't have a lot of spare time at the moment but will take a look when I can. In the meantime, if you - or anyone else - wants to work on this and submit a PR, that is fine