Semantic convention tool should lint the enum values

[rakyll]

We would like to enforce formatting for enum values in semantic conventions. (See open-telemetry/opentelemetry-specification#1519 for more details.)

In order to enforce the requirements of only allowing lower case characters and underscores (_), we need linting rules in the markdown generator. The generator should error if there are enum values that doesn't meet this criteria.

[Oberon00]

The tool should also lint enum ids (and actually all ids), see #59

[Oberon00]

I'm working on this

Actually I'll only validate IDs for now (i.e. #59). We already have enums that do not conform to the current rule (namely http.flavor=1.0 or 2.0). So we'd need to discuss that first or provide an opt-out for the check.