Simplify Session.render()

[josiah-wolf-oberholtzer]

The renderable dependency graph system is cool, and was a great learning experience, but in practice it's much too complex for most use cases.

We can simplify Session.render() so that it supports all common use-cases:

• Render a session to an output file
• Render a session but without an output file (e.g. generating analysis as a side-effect)
• Render a session with an input file and an output file
• Render a session but with all audio files (input/output/buffers) referenced as relative paths

More complex dependency graph logic can be reimplemented on top of a simpler render system.

[josiah-wolf-oberholtzer]

The multiple-renders-in-one approach might still be feasible, but:

• The path handling inside SessionRenderer should be rewritten because it's very confusing
• The render.yml output has never seen use and should be removed
• Renders should default to a temp directory as their render directory

[josiah-wolf-oberholtzer]

Writing to /dev/null (on Linux/OSX) and NUL (on Windows) should be made simple.

[josiah-wolf-oberholtzer]

Done in #287