Support adding priorities for Platform::addAssetDirectory()

[lucasvickers]

This would allow us to prioritize which folder is searched first in the following function:
https://github.com/cinder/Cinder/blob/master/src/cinder/app/Platform.cpp#L119

The back story: with cmake a binary can be built in a different folder hierarchy than the assets folder. If you add that folder to the search path using AddAssetDirectory, then move the binary adjacent to a different assets folder, there could be some user ambiguity over which folder is being accessed. This would at least allow the user to explicitly state which folder they want to prioritize.

[richardeakin]

One option would be to offer the ability to prepend directories, as addAssetDirectory() currently only appends them. And another would be to store the directories in a std::map<int, fs::path*, std::greater<int> > data struct, similar to how we handle priorities with signals.