new "existing" option in docs

[Stephen Mallette <sp...@gmail.com>]

To this point we could do:

[gremlin-groovy,modern]
----
g.V()
----

in asciidoc to get the modern graph loaded as an example. A problem I
frequently found in writing docs though was that when I wanted to use a
"custom" graph i wasn't able to because the state of the graph would reset
in the next [gremlin-groovy] tag. That led to big fat code blocks with
call-outs which wasn't always nice.

With some help from kuppitz, we now have this option:

[gremlin-groovy,existing]

With this tag, the state of the previous console is maintained and you can
build longer run examples across multiple tags. I think that will help make
the docs more fluid and allow us to write better quality examples for when
the toy graphs don't cut it.

This is only implemented as of 3.2.3 and not on the 3.1.x line (it would be
trivial to add, but we aren't really writing any new docs over there, so I
don't think it's a problem).