Let projects control arguments passed to the eproject-tags-etags command

This commit allows the following:

    (define-project-type foo (generic)
      (look-for "foo.cfg")
      :tags-cmd-args (lambda (root) '("-R" "--languages=-JavaScript" "my/files")))

which - when eproject-tags is called from a foo project - will invoke etags with:

    etags -o .TAGS-tmp --verbose -R --languages=-JavaScript my/files