Add InferLevelsWithTimestamp option

[lgfa29]

This PR adds a new option called InferLevelsWithTimestamp to the StandardLoggerOptions that allow ignoring timestamps from the beginning of log lines before attempting to infer log level.

Timestamp detection is done using a simple regular expression that looks for characters commonly found in timetamps, such as digits, :, / etc.

A new option was added to avoid modifying the existing InferLevels options. An alternative approach that I thought about was to add a InferLevelsOptions attribute to control how infer levels work that could a new struct or a bit mask:

// As bit flags
appLogger.StandardWriter(&hclog.StandardLoggerOptions{
  InferLevels: true,
  InferLevelsOptions: hclog.IgnoreTimestamps
})

// As struct
appLogger.StandardWriter(&hclog.StandardLoggerOptions{
  InferLevels: true,
  InferLevelsOptions: &hclog.InferLevelsOptions{
    IgnoreTimestamps: true,
  }
})

But I'm not sure if this approach is better, so I'm open for feedback 🙂

Closes #100