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`.

docs/cli.md

@@ -20,5 +20,5 @@ $ wasmtime --invoke _start foo.wasm
 ```
 
 For more information be sure to check out [how to install the
-CLI](cli-install.md) as well as [the list of options you can
-pass](cli-options.md).
+CLI](cli-install.md), [the list of options you can
+pass](cli-options.md), and [how to enable logging](cli-logging.md).