* Add `benchmark_context` flag that allows per-run custom context.
Add support for key-value flags in general.
Added test for key-value flags.
Added `benchmark_context` flag.
Output content of `benchmark_context` to base reporter.
Solves the first part of #525.
* Docs and better help
As noted in #995, this causes issues when the command line flag already
starts with "benchmark_", which they all do.
Not caught by tests as the test flags didn't start with "benchmark".
Fixes#995
Initialize option flags from environment variables values if they are defined, eg. `BENCHMARK_OUT=<filename>` for `--benchmark_out=<filename>`. Command line flag value always prevails.
Fixes https://github.com/google/benchmark/issues/881.
Dominic Hamon