Unnecessarily exported identifiers inspection

[zolotov]

The same funtionality as implemented in https://github.com/isaiah/unexport

[dlsniper]

I think this inspection could work only if the project inspected is an application not a library. And maybe it should be turned off by default or a WEAK_WARNING.

[zolotov]

@dlsniper indeed, it could be weak_warning by default or enabled for not_on_the_fly_mode only