allow to supress the warning about old database version

src/old.rs

@@ -72,6 +72,7 @@ pub fn time_or_warning<D: Database>(time: DateTime<Utc>, db: &D) -> Result<(Date
 /// emits the appropiate warning if the old database format was detected.
 pub fn warn_if_needed<E: Write>(err: &mut E, needs_warning: bool) -> Result<()> {
     if needs_warning {
+        if std::env::var_os("TIEMPO_SUPRESS_TIMETRAP_WARNING").is_none() {
             writeln!(
                 err,
                 "{} You are using the old timetrap format, it is advised that \
@@ -79,6 +80,7 @@ pub fn warn_if_needed<E: Write>(err: &mut E, needs_warning: bool) -> Result<()>
                 Yellow.bold().paint("[WARNING]"),
             ).map_err(|e| IOError(e))?;
         }
+    }
 
     Ok(())
 }