More flexibility to create_schema_graph

[chunweiyuan]

On my own fork I customized the code a little for our own aesthetic needs:
https://github.com/chunweiyuan/sqlalchemy_schemadisplay/blob/foreign_key_edges/sqlalchemy_schemadisplay.py#L171-L203

However, it does feel like we should avoid this type of hard-coding and allow create_schema_graph's API to take arguments for graphical tuning.

1.) Is there any plan to make create_schema_graph more flexible for plotting?
2.) Running pytest in python 3.6 fails due to issues with StringIO and cStringIO. Should we make this thing 2/3 compatible?

[abitrolly]

@chunweiyuan can you post a screenshot of your output here?

2. is addressed by Fixing error with python 3 #14