default StylesPath

[paper42]

I think it would be beneficial to have StylesPath set to a specific string if the user did not overwrite it with a config file. Default could be /usr/share/vale/, but it should be a compile time option. This would allow Linux distributions to ship styles without the user having to figure out where the styles were installed.

[bollwyvl]

As mentioned over on #576, the XDG spec provides some useful, not-having-an-adventure norms for where to put such things, even on windows, with existing go implementations (and many other languages).