chromium: Allow env vars for passing plugin paths.

Introduces environment variables to set plugin base paths. The schema
for these is like NIX_CHROMIUM_PLUGIN_PATH_<N>. Where <N> is the path
type we want to change, the supported (full) variable names are:

 * NIX_CHROMIUM_PLUGIN_PATH_ALL
 * NIX_CHROMIUM_PLUGIN_PATH_PEPPERFLASH
 * NIX_CHROMIUM_PLUGIN_PATH_FILEFLASH
 * NIX_CHROMIUM_PLUGIN_PATH_PDF
 * NIX_CHROMIUM_PLUGIN_PATH_FILE_EFFECTS
 * NIX_CHROMIUM_PLUGIN_PATH_NACL
 * NIX_CHROMIUM_PLUGIN_PATH_PNACL
 * NIX_CHROMIUM_PLUGIN_PATH_WIDEVINE

Whereas NIX_CHROMIUM_PLUGIN_PATH_ALL is the plugin base path for every
path which is not set explicitly, so by setting ..._ALL and not setting
..._WIDEVINE, the widevine plugin will be searched in the directory
specified using ..._ALL.

Right now, the only plugin where this is used is widevine, and it still
doesn't properly work yet.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>