Enhanced to allow control over the start index

In my project [SNMP4S](http://snmp4s.org) I needed sbt-boilerplate to skip the i = 1 iteration.  I enhanced my fork of sbt-boilerplate to allow specifying the start index (In my case, I set it to 2).  This is what it looks like:

`[2#abc ##1 # ++ ]`

Of course, if you omit the `2`, the behavior is as before.  Let me know if you like this enhancement.  I can update the README as well.  

Thanks for building such a handy plugin!

-Joe