Fixes#10.
Adds a mode that forces a benchmark to only run once, simplifying
profiling usecases. This can be enabled by any of the following methods:
* Passing `--run-once` on the command line
* `NVBENCH_CREATE(...).set_run_once(true)` when declaring a benchmark
* `state.set_run_once(true)` from within the benchmark implementation.
Human-readable outputs (md) and CLI inputs still use percentages.
In-memory and machine-readable outputs (csv, json) use ratios.
This is the convention that spreadsheet apps expect. Fixes#2.
