register_application/1 first version

I added support for some new entries in the `exometer_predefined` scripts:

* `{delete, Key}` deletes a specific entry
* `{select_delete, Pattern}` deletes all entries matching the select pattern
* `{re_register, {Name, Type, Opts}}` adds or redefines an entry.

The functions `exometer:register_application()` and `exometer:register_application(App)` are meant to be called during application start (e.g. in a start phase hook). The function does nothing if exometer isn't running, but otherwise checks and loads the predefined settings. This is useful for applications that are loaded and started after exometer (note that if they are loaded from start, their predefines will be picked up.

If `exometer:register_application(App)` is called again, it will re-check and re-load. This can be used e.g. during application upgrade.