User settings in it's own file

[p3lim]

Currently you've designed user settings to be added in the default Preferences.sublime-settings file, but you should not advertise that.

Since this is a plugin, it can also use the file /Packages/User/ShellExec.sublime-settings to get it's settings, and this is the correct way to do it as well.

Moving over to the plugin-specific settings file allow you to use shorter names on the configuration options as well.