Add option to make the leading stars invisible

[lmintmate]

This PR adds the option to make the leading stars completely invisible, and not just hidden by means of color, via the variable org-bullets-invisible-leading-stars (nil by default).
In particular, when this variable is set to t, the headings will look like

x Heading level 1
o Heading level 2
> Heading level 3
o Heading level 2

instead of

x Heading level 1
 o Heading level 2
  > Heading level 3
 o Heading level 2

This is useful if the contents of org-bullets-bullet-list make obvious the level of each heading, and thus indentation in that case is superfluous.
Note that the code of this PR originally came from sabof#13, and, as the original PR modified a part of the minor mode definition, which was moved to the org-bullets--keywords variable instead in the emacsorphanage version, a modification to move some code in its new proper place had to be done in comparison to the original PR.

P.S. I've been using a forked version of org-bullets with this code added in my own emacs config for about a month without problems, and thus thought that it would be nice if it could be added to the upstream emacsorphanage repo, so that others can benefit from it.

[tarsius]

Thanks!

I however had to make the changes to this
file by hand because the emacsorphanage version has added most of the
stuff that used to be in the minor mode definition in the
org-bullets--keywords variable instead and there would thus be a
conflict if I attempted to merge the specific commits.

I have more experience dealing with conflicts, so I have redone this while preserving the original commits (well the messages and authorship information). I have pushed the result to the invisible branch

Before merging that I want to improve the doc-string and think about changing the name of the option.

[tarsius]

I am afraid this conflicts with with sabof#19. I have pushed a version of that to the pr-19-without-cosmetics branch.