Update task to add methods_only parameter

I want to use flog in my build with the rake task and set a max score per method, but it's currently failing because main#none has a ridiculously high score. 

I found that I can use the flag -m on the command line but it wasn't available from the rake task. I've just added a parameter defaulted to false, seemed the simplest thing but let me know if you have a better suggestion.