Syntax-driven line-continuation in the graph

[hjoliver]

Supersedes #1328. We're keeping the standard line-continuation backslash for the moment, but we should make it entirely unnecessary in the graph.

I.e., as well as this:

graph = """   # (already supported)
   foo => bar => \
       baz
"""

and this:

graph = """   # (already supported)
   foo => bar
       bar => baz
"""

we should also allow this:

graph = """"
   foo => bar =>
      baz
"""