Add option to auto-use --jobs

[Flamefire]

Git 2.8.0 added the --jobs param to fetch submodules in parallel. E.g. git submodule -q update --init --jobs 3 libs/config libs/headers libs/../tools/boost_install libs/../tools/build libs/../tools/cmake

Would be great if boostdep had an option to auto-use that when possible. Checking the git version in Bash is a bit cumbersome and using --git_args "--jobs 3" is leading to errors for older git versions