Allow ability to specify axes scales

[ffarhour]

Implement --yscale and --xscale command arguments thaat add scale such as log or linear to the vega-lite spec.
This can be achieved by using Issue #21 : the mechhanism to merge 2 JSONs.

The --xscale and --yscale would generate a "wedge.json" which could then be merged with the main vega-lite json generated from the inputs.