Add store-too functionality for magic commands

[austinkline]

Issue #, if available: #17

Description of changes:

• Add a parameter to many line and cell magics to allow the query result to be stored to a specified variable. Syntax:

# this is a notebook cell
%%gremlin --store-to gremlin_res

g.V().limit(1)

# this is another notebook cell executed after the first
print(gremlin_res)

[v[some-id-here]]

If a cell throws an exception, we will populate the variable graph_notebook_error with the exception. For Example:

# this is a notebook cell
%%gremlin 

g.V().invalidStep()

# this is another notebook cell
print(graph_notebook_error)

499: {"code":"MalformedQueryException","requestId":"...","detailedMessage":"Query parsing failed at line 2, character position at 13, error message : token recognition error at: 'St'"}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[krlawrence]

Is this something we will expose to users?

[austinkline]

It is not documented yet, but we should add this to our README

[krlawrence]

I left one comment as a question on the GRAPH_NOTEBOOK_CONFIG env var.