* 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
Bátor Tallér