Log to stderr by default. (#1266)

Change the default from file-per-thread-logger to pretty-env-logger,
which is more common in Rust projects, and change the option from `-d`
to `--log-to-files`.

src/commands/run.rs

@@ -89,11 +89,11 @@ pub struct RunCommand {
 impl RunCommand {
     /// Executes the command.
     pub fn execute(&self) -> Result<()> {
-        if self.common.debug {
-            pretty_env_logger::init();
-        } else {
+        if self.common.log_to_files {
             let prefix = "wasmtime.dbg.";
             init_file_per_thread_logger(prefix);
+        } else {
+            pretty_env_logger::init();
         }
 
         let config = self.common.config()?;