Pad concat fragment order numbers with 0 in vhost

Since fragments are concatenated in the order the shell sees them (at
least on non-Windows systems) an order number of 180 takes precedence
over an order number of 20, even though 20 should come before 180. This
goes against the intention behind specifying order numbers.

Padding order numbers <100 with a leading 0 restores the intended and
correct order in which fragments are concatenated.

Includes small update to the related README section. Fixes MODULES-1348.