Added support for conditional var assignments.
Added support for conditional var assignments.

FOO?="BAR" will only assign "BAR" to $FOO if the $FOO variable is empty.

nodenv/nodenv-vars,dark5un/nodenv-vars,johnbellone/nodenv-vars,pvdb/rbenv-vars,sgonyea/rbenv-vars,dpayonk/rbenv-env,sstephenson/rbenv-vars,OiNutter/nodenv-vars,cehoffman/luaenv-vars,rbenv/rbenv-vars