Generate graph edge costs based on measured performance

The `cost` for odo's graph isn't generated on the actual runtime cost of the conversion, which leads odo to [choose a conversion path that is sub-optimal](https://github.com/blaze/blaze/issues/1235).

To the extent that an edge's cost accurately reflects its actual runtime, the better odo can choose an optimal path from one format to another.

To generate these costs requires representative data sources for the different backends and a script that runs odo's basic converters for each backend pairing. To be more accurate, the script would need to run on differently sized data and on different platforms, but even just a single platform run with decently sized data sources will improve the current heuristic costs.

These generated costs would be generated by the script and stored for usage at runtime. Odo will need to be refactored to read in these generated costs and apply them to the graph edges.

@cpcloud thoughts?