Enable use of Logger as a filter

[jethrogb]

The spec parsing and matching of env_logger is very useful, but it will only let you log to stdout/stderr. With this change you can embed env_logger's logic in your own logger.

[sebasmagri]

@jethrogb it looks good to me initially. However, could you please add an example or a few documentation lines explaining how to use this with your own logger?

[jethrogb]

Added docs

[sebasmagri]

Hi @jethrogb!

Could you please review and rebase changes against the latest master to merge this?

[jethrogb]

Rebased

[jethrogb]

Rebased, again. Can you please either accept or decline this PR so I don't have to keep rebasing it?

[KodrAus]

Thanks for this @jethrogb! Sorry about the delay, I'll give this a proper review this week. You won't need to rebase again.

[KodrAus]

Thanks again for doing this @jethrogb! I think this is a good start and will merge it in once the build is green. We might want to add some dedicated APIs over the builder before the next release so you can parse a filter for your own loggers without having to set a target explicitly.

[KodrAus]

Thanks again for doing this @jethrogb! I think this is a good start and will merge it in once the build is green. We might want to add some dedicated APIs over the builder before the next release so you can parse a filter for your own loggers without having to set a target explicitly.

[jethrogb]

once the build is green

Fixed. Probably should've run cargo build after the rebase.

without having to set a target explicitly

You don't have to set it. If you only call EnvLogger::matches, the target is never used.

[KodrAus]

You don't have to set it. If you only call EnvLogger::matches, the target is never used.

Ah you're right. Having a silent target is a prerequisite to anything else we'd want to do here anyway so there's no reason to block this on thinking about other ways to expose the filtering.

Thanks again!