slog-envlogger 0.4.0

# slog-envlogger - Port of `env_logger` as a `slog-rs` drain

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  <strong><a href="//dpc.github.io/slog-envlogger/">Documentation</a></strong>
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`env_logger` is a de facto standard Rust logger implementation, which allows
controlling logging to `stderr` via `RUST_LOG` environment variable.

This is a fork of `env_logger` that makes it work as a `slog-rs` drain:

Notable changes:

* Support for `slog-stdlog` to provide support for legacy `info!(...)` like
  statements.
* `envlogger` does not do formatting anymore: `slog-envlogger` can be composed
  with any other `slog-rs` drains, so there's no point for it to provide it's
  own formatting. You can now output JSON to a file, controlling it via
  `RUST_LOG` environment var. `envlogger::init()` is provided for convenience
  doing formatting to `stderr`

### Status & news

**Warning**: Documentation has been been left mostly untouched, which means some
places of it might be confusing.

### How to use

See `examples` directory.

The simplest way to convert existing project to use `slog-rs`+`slog-envlogger` is:

```rust
fn main() {
    slog_envlogger::init().unwrap();

    error!("error");
    info!("info");
    trace!("trace");
}
```

More proper (and powerful) version would be:

```rust
fn main() {
    let term = slog_term::stderr();
    let drain = slog_envlogger::new(term);

    let root_logger = drain.into_logger(o!("build" => "8jdkj2df", "version" => "0.1.5"));

    slog_stdlog::set_logger(root_logger.clone()).unwrap();

    slog_error!(root_logger, "slog error");
    error!("log error");
    slog_info!(root_logger, "slog info");
    info!("log info");
    slog_trace!(root_logger, "slog trace");
    trace!("log trace");
}
```

Using `slog-stdlog` scopes you can make parts of the code log additional information (see [`scopes` example][scopes]):

[scopes]: https://github.com/dpc/slog-envlogger/blob/master/examples/scopes.rs

```rust
fn main() {
    slog_envlogger::init().unwrap();

    error!("log error");

    slog_stdlog::scope(
        slog_stdlog::with_current_logger(|l| l.new(o!("scope-extra-data" => "data"))),
        || foo()
    );

    trace!("log trace");
}

fn foo() {
    info!("log info inside foo");

    // scopes can be nested!
    slog_stdlog::scope(
        slog_stdlog::with_current_logger(|l| l.new(o!("even-more-scope-extra-data" => "data2"))),
        || bar()
    );
}

fn bar() {
    info!("log info inside bar");
}
```