Add a Funnel view

[nowthis]

Sankeys are sometimes used to show funnels, i.e. flows where you start with a large population and monotonically decrease the amounts as a process moves through phases.

These sorts of graphs can be made easier than SankeyMATIC currently makes them, possibly with a simplified syntax.

Related:

• Issue #4 which would let you display a funnel vertically.
• Issue #16 would let you center each chunk of the funnel, making it look much better.