flow-tool, a new tool to run Flow scripts standalone (basically just for unit-testing, however)

Change-Id: I79b913cacd3f76594c5a6cd752c5a89179245808
[flow] FlowRunner: add onParseComplete event hook

Change-Id: Ia878702356d00fb77e62da0d3589b1f519ddd9ae
flow-tool: work-in-progress improvements

Change-Id: I3a229f20b3809141755aeb53db55284642821f4c