Validate values of benchmark configuration options

Currently, some config values are validated in :plugin (e.g., `reportFormat`), while some others are validated in :runtime (e.g., `iterationTimeUnit`). Others aren't validated at all, e.g., `iterations` (validated only on JVM by JMH).
All config values should be validated in :plugin.

Examples of invalid config values:
```
iterations = -10
iterationTimeUnit = "yes"
```