Add support for custom extensions, rewrite with optparse

Hi! I was using herbgobbler to prepare one of my projects for localization, but I ran into the issue of not being able to use custom extensions. I prefer to use .html, rather than .html.erb (not exactly standard, but I prefer the shorter extension). Unfortunately, herbgobbler didn't support custom extensions, so I went ahead and added support for that.

Along with that, I rewrote the gobbler executable to use optparse, which makes it much easier to add more arguments if there is ever a need to do so. 

I also corrected the tr8n link in the README.

I tested this thoroughly,  but testing it some more wouldn't be a bad idea.

Thanks!