Expose an environment variable for config file lookup

[chapulina]

We currently only support looking up config files by absolute paths, or paths relative to the running directory. It would be convenient for downstream packages if users could reference their installed configs without knowing the full path.

Desired behavior

1. A package installs several config files in, for example, install/share/package_name/gui_config
2. Then it sets the IGN_GUI_CONFIG_PATH variable to that folder
3. At runtime, users can load those config files from the command line with ign gui -c filename.config or ign gazebo --gui-config filename.config

Alternatives considered

We usually use environment variables for this kind of lookup path. One thing to consider is to have a generic path, such as IGN_GUI_RESOURCES, that can be reused for things other than config files.

Implementation suggestion

Look at how IGN_GAZEBO_RESOURCE_PATH is implemented, for example.