feature suggestion: parallel support

[myyan92]

with 3-4 lines of modification in io.py we can enable parallel rendering with python's multiprocessing.Pool.

currently the povstring is written to temp.pov, just insert a randomly generated string would make it safe to multiprocessing.

[blackyang]

In the render_povstring() function in io.py, there is an argument called tempfile, so you can set it yourself for parallel support

[JBlaschke]

The tempfile kwarg is not available in the pip version of Vapory (see: #40). I.e. if you use pip to install vapory, this won't work. It would be nice if @Zulko would update the pip version....

[tabenius]

Would be especially nice to get to work with MoviePy, rendering several frames at a time with different 't'. Anyone know how?