Add FlowChart diagram type

Add a custom diagram type for working with FlowCharts.

Sub classes should include:
- Start
- End
- Process
- Decision
Each class should have the appropriate functions for linking. FlowChart should have an .auto_layout() function that will attempt to make it pretty.

No files or tests are named. Start by reading the existing diagram types and their linking APIs, then inspect how layout is represented. Done means adding FlowChart with Start, End, Process, and Decision subclasses, appropriate linking functions, and an auto_layout() function that produces a presentable arrangement.

Written by the indexing model from the issue text.