Plugin autoloading for Linux

Read ~/.config/vapoursynth/vapoursynth.conf or
/etc/xdg/vapoursynth/vapoursynth.conf.

The default config file would look like this:
AutoloadSystemPluginDir=true
AutoloadUserPluginDir=true

; set at 'waf configure'
SystemPluginDir=$plugindir
; empty, thus not used
UserPluginDir=