Allow sub-directories in Fixture folder

It can be very painful to have all fixtures in one directory.
I think it would be great to be able to create sub-directories for a better file organisation.

Currently, it is 'app.my_fixture' for Fixture/MyFixtureFixture.php

If you want to add n subdirectories, you can by doing so :
'app.myFolder.mySecFolder.myThirdFolder.my_fixture' 
for 
Fixture/myFolder/mySecFolder/myThirdFolder/MyFixtureFixture.php