doc: Clarify the meaning of VFMMics

Also other tweaks to VFM's documentation.
VFM: Fix the order of the elements of VFMMics
VFM: Switch to fmParallel

In my tests, mode 5 with default parameters is ~20% faster.
Mode 5 with micmatch 2 is ~55% faster.
doc: Don't suggest specifically version 3.3 of Python
doc: Document plugin autoloading in Linux and OS X
doc: Show VapourSynth's version instead of "1.0"
doc: Mention the length of DoubleWeave's output

Also include an example.
doc: PlaneDifference accepts clips of unequal lengths
stdlib: Add DuplicateFrames, DeleteFrames, FreezeFrames

And their documentation.
doc: Python keywords shouldn't be namespaces
VFM: Use PlaneDifference to detect scene changes

This way each plane difference is calculated only once.