[O] allow multiline #+BIND

[Vladimir Alexiev]

It would be nice to allow multiline BINDs. I'd much rather write this than put 
it all on one line:

#+BIND: va/org-dot-preamble "digraph g {
#+BIND:   rankdir=LR nodesep=0.2 ranksep=0.1 arrowsize=0.2
#+BIND:   node [fontname=courier fontsize=8 margin='0.02,0.01' shape=circle 
width=0.1 height=0.1 label='']
#+BIND:   edge [fontname=courier fontsize=8 labelfontname=courier 
labelfontsize=8]"

The usual continuation syntax of backslash before the EOL can be used.
Or else, a new keyword #+BIND1: for each continuation line.

The function to fix is ox.el::org-export--list-bound-variables