benchmark

Allow setting the default time unit globally (#1337)

* Add option to set the default time unit globally

This commit introduces the `--benchmark_time_unit={ns|us|ms|s}` command line argument. The argument only affects benchmarks where the time unit is not set explicitly.

* Update AUTHORS and CONTRIBUTORS

* Test `SetDefaultTimeUnit`

* clang format

* Use `GetDefaultTimeUnit()` for initializing `TimeUnit` variables

* Review fixes

* Export functions

* Add comment