Add cli option to set the verbosity level

The verbosity was set either to info (in release mode) or debug (in
debug mode).

Add a command-line argument to change it, so that users can enable debug
logs using the release:

    scrcpy -Vdebug

app/scrcpy.1

@@ -154,6 +154,12 @@ It only shows physical touches (not clicks from scrcpy).
 .B \-v, \-\-version
 Print the version of scrcpy.
 
+.TP
+.BI "\-V, \-\-verbosity " value
+Set the log level ("debug", "info", "warn" or "error").
+
+Default is "info" for release builds, "debug" for debug builds.
+
 .TP
 .B \-w, \-\-stay-awake
 Keep the device on while scrcpy is running.