New startup options, show<n>levels

[Gustav Wikström]

Prompted by a question on StackOverflow, 
https://stackoverflow.com/questions/56536184/set-initial-visiblity-to-a-certain-level-in-org-mode,
 a few new options are added to the startup setting.

Patch is applied to master as this is non-critical and it is communicated here 
and now for full transparency. See commit hash a71ac14e4, 
https://code.orgmode.org/bzg/org-mode/commit/a71ac14e46bb820abdbd2e6651c58179c50eb2fa

Hope these new options will be usable for some of you!

Kind Regards
Gustav