Add "Log messages in a custom format" recipe

src/app.md

@@ -5,6 +5,7 @@
 | [Parse command line arguments][ex-clap-basic] | [![clap-badge]][clap] | [![cat-command-line-badge]][cat-command-line] |
 | [Log a debug message to the console][ex-log-debug] | [![log-badge]][log] [![env_logger-badge]][env_logger] | [![cat-debugging-badge]][cat-debugging] |
 | [Log an error message to the console][ex-log-error] | [![log-badge]][log] [![env_logger-badge]][env_logger] | [![cat-debugging-badge]][cat-debugging] |
+| [Log messages in a custom format][ex-log-custom] | [![log-badge]][log] | [![cat-debugging-badge]][cat-debugging] |
 | [Enable log levels per module][ex-log-mod] | [![log-badge]][log] [![env_logger-badge]][env_logger] | [![cat-debugging-badge]][cat-debugging] |
 | [Log to the Unix syslog][ex-log-syslog] | [![log-badge]][log] [![syslog-badge]][syslog] | [![cat-debugging-badge]][cat-debugging] |
 | [Log messages to a custom location][ex-log-custom] | [![log-badge]][log] | [![cat-debugging-badge]][cat-debugging] |
@@ -120,6 +121,51 @@ Your favorite number must be 256.
 
 [Write me!](https://github.com/brson/rust-cookbook/issues/61)
 
+[ex-log-custom]: #ex-log-custom
+<a name="ex-log-custom"></a>
+## Log messages in a custom format
+
+[![log-badge]][log] [![cat-debugging-badge]][cat-debugging]
+
+Set a basic custom logger `ConsoleLogger` and log various messages to it.
+
+```rust
+#[macro_use]
+extern crate log;
+
+use log::{LogRecord, LogLevel, LogMetadata, LogLevelFilter, SetLoggerError};
+
+struct ConsoleLogger;
+
+impl log::Log for ConsoleLogger {
+    fn enabled(&self, metadata: &LogMetadata) -> bool {
+        metadata.level() <= LogLevel::Info
+    }
+
+    fn log(&self, record: &LogRecord) {
+        if self.enabled(record.metadata()) {
+            println!("Rust says: {} - {}", record.level(), record.args());
+        }
+    }
+}
+
+fn run() -> Result<(), SetLoggerError> {
+    log::set_logger(|max_log_level| {
+                        max_log_level.set(LogLevelFilter::Info);
+                        Box::new(ConsoleLogger)
+                    })?;
+    
+    info!("hello log");
+    warn!("warning");
+    error!("oops");
+    Ok(())
+}
+
+fn main() {
+    run().unwrap();
+}
+```
+
 [ex-log-mod]: #ex-log-mod
 <a name="ex-log-mod"></a>
 ## Enable log levels per module