Add support for dot notation when accessing custom keys

[dror-weiss]

Hello,

In Pino, I'm using its nestedKey feature with a custom time field that I want Pino-Pretty will parse. However, the --timestampKey supports only fields in the root of the log entry.

I would like to provide --timestampKey nested_key.time to be used as the time value.

This is also relevant to all custom keys the library allows to override.

[mcollina]

Thanks for reporting! Would you like to send a Pull Request to address this issue? Remember to add unit tests.