Lot's of fixes to image loading and saving thanks to @ukoethe

• Allow image IO for value_types other than uint8_t
• dump_image(): check if data has a channel axis
• fixed shape in load_image()/dump_image()

Supress warnings in cling

Fix: Setting library directory for cling with imported config.hpp file.

• Add audio reading and writing with libsndfile (supports WAV and OGG files)
• Add documentation

The first release of xtensor-io is here. Load some crispy PNGs or even JPEG's. And saving is possible, too!
Also available: Reading and writing of NumPy compressed NPZ files, in C++.

All features still have to be battle-tested, and we're happy to add some more options on demand (for example for image loading).