Support more than two digits of each unit

Now it's possible to do `countdown 100m` or `countdown 120s`.

src/countdown/__main__.py

@@ -17,11 +17,11 @@
     r"""
     ^
     (?:                 # Optional minutes
-        ( \d{1,2} )     # D or DD
+        ( \d+ )         # one or more digits
         m               # "m"
     )?
     (?:                 # Optional seconds
-        ( \d{1,2} )     # D or DD
+        ( \d+ )         # one or more digits
         s               # "s"
     )?
     $