allow to supress the warning about old database version

src/old.rs

@@ -72,12 +72,14 @@ 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 {
-        writeln!(
-            err,
-            "{} You are using the old timetrap format, it is advised that \
-            you update your database using t migrate",
-            Yellow.bold().paint("[WARNING]"),
-        ).map_err(|e| IOError(e))?;
+        if std::env::var_os("TIEMPO_SUPRESS_TIMETRAP_WARNING").is_none() {
+            writeln!(
+                err,
+                "{} You are using the old timetrap format, it is advised that \
+                you update your database using t migrate",
+                Yellow.bold().paint("[WARNING]"),
+            ).map_err(|e| IOError(e))?;
+        }
     }
 
     Ok(())