favicon_base.png not being removed from build dir

From issue #17: https://github.com/follmann/middleman-favicon-maker/issues/17
Using an incredibly complex technique I call 'putting an underscore in front of the filename' I was able to solve the problem. :wink:
I therefore propose changing the default filename, this way there is no code needed to remove the file and it just fits with middleman's architecture.
Untested, but should work.