docs: Add example for setting default log level

A new example, `log_level.rs`, demonstrates how to set the default log
level for Verbosity to something other than the default. It can be run
with `cargo run --example=log_level [default level] [flags]`. E.g.,:
`cargo run --example=log_level off -vvvv` will set the default log level
to `OffLevel` and then apply the verbosity flags to set the log level to
`Debug`.

examples/log_level.rs

@@ -0,0 +1,67 @@
+//! Demonstrates how to set the default log level for the logger to something other than the default
+//! (`ErrorLevel`). This is done with multiple subcommands, each with their own verbosity level.
+
+use clap::{Parser, Subcommand};
+use clap_verbosity_flag::{
+    DebugLevel, ErrorLevel, InfoLevel, OffLevel, TraceLevel, Verbosity, WarnLevel,
+};
+
+#[derive(Debug, Parser)]
+struct Cli {
+    #[command(subcommand)]
+    command: Command,
+}
+
+#[derive(Debug, Subcommand)]
+enum Command {
+    Off {
+        #[command(flatten)]
+        verbose: Verbosity<OffLevel>,
+    },
+    Error {
+        #[command(flatten)]
+        verbose: Verbosity<ErrorLevel>,
+    },
+    Warn {
+        #[command(flatten)]
+        verbose: Verbosity<WarnLevel>,
+    },
+    Info {
+        #[command(flatten)]
+        verbose: Verbosity<InfoLevel>,
+    },
+    Debug {
+        #[command(flatten)]
+        verbose: Verbosity<DebugLevel>,
+    },
+    Trace {
+        #[command(flatten)]
+        verbose: Verbosity<TraceLevel>,
+    },
+}
+
+impl Command {
+    fn log_level_filter(&self) -> log::LevelFilter {
+        match self {
+            Command::Off { verbose } => verbose.log_level_filter(),
+            Command::Error { verbose } => verbose.log_level_filter(),
+            Command::Warn { verbose } => verbose.log_level_filter(),
+            Command::Info { verbose } => verbose.log_level_filter(),
+            Command::Debug { verbose } => verbose.log_level_filter(),
+            Command::Trace { verbose } => verbose.log_level_filter(),
+        }
+    }
+}
+
+fn main() {
+    let cli = Cli::parse();
+    env_logger::Builder::new()
+        .filter_level(cli.command.log_level_filter())
+        .init();
+
+    log::error!("Engines exploded");
+    log::warn!("Engines smoking");
+    log::info!("Engines exist");
+    log::debug!("Engine temperature is 200 degrees");
+    log::trace!("Engine subsection is 300 degrees");
+}