Corrected wrapper entries:
- Need to override vapoursynth with `vapoursynth.withPlugins` for av1an to be able to use VS plugins. Changed PYTHON_PATH accordingly. Passing `pkgs.vapoursynth` to `python3Packages.makePythonPath` makes an incorrect path entry, resulting in segfault when the program is run.
- Need LD_LIBRARY_PATH to use VS plugins (e.g. ffms2 and lsmash, the recommended chunking method). Otherwise get e.g. `Error: FFMS2 is not installed, but it was specified as the chunk method`.
- Need vapoursynth in PATH to use VS based chunking methods or VS script as input. At least, `vspipe` binary needs to be visible in PATH.
Add mkvtoolnix option; it is the recommended concatenation method.
