Git v2.8 introduced support for parallel fetching of submodules. This
can result in a substantial performance boost when performing a "git
submodule update" in a repository with many submodules, potentially
further magnified when updating submodules recursively.
This commit introduces a check on the Git version to see if it supports
parallel submodule fetching. If it does, we add the '--jobs=N' argument
to our call to "git submodule update", where N is g:plug_threads or the
vim-plug default of 16.
