New, expanded DefaultParamMeaning

[rrnewton]

When it's done, this should address issue #73.

Please discuss here or on that issue. The basic steps are:

• Expand DefaultParamMeaning so it can say more things (like setting a field in a custom column)
• Create a uniform way to interpret param meanings into "setters" for BenchmarkResult
• Refactor the main "App.hs" to rely on this rather than the internal details of DefaultParamMeaning