Handle graph-definition syntax for edges

[rskew]

Upstream PR: geekscape#27

Example graph:

   __________
  /   \      \
 A     B ---- C --->
  \___/______/

has syntax in a pipeline definition:

  "graph": [
    "(A B (A.a_out_1: b_in_1 A.a_out_2: b_in_2) C (A.a_out_1: c_in_1 B.b_out_1: c_in_2 A.a_out_2: c_in_3))"
  ],

Note that output names must be fully-qualified, e.g. "B.b_out_1" instead
of "b_out_1". This is due to the graph traversal not yet handling edges
defined between B and C in the example graph, only between A and B, and
between A and C.

[tall-josh]

Nice, I was also hitting up against the issue of input/output name mismatches.

Another thing to consider (out of scope of this PR) is the ability to access items from an output object. For example, if a Capability outputs a DataFile with image .content then it cannot be used by a PipelineElement that acceps image input. At the moment we'd need to put a shim between the Capability and PipelineElement that does the unpacking. If we could move that to the framework maybe that's a good thing