Recursive dependencies

Usually when using stylus, there is a main file e.g. `app.styl` that imports another files like `_header.styl`, `_footer.styl`,... These two files are dependencies of the former one, so when one of this files changes, brunch watch recompile `app.styl`. 

The problem comes when one of this dependencies has another dependency. For example, imagine that `_header.styl` imports `_menu.styl`. In this case, if `_menu.styl` changes, `app.styl` is not recompiled by brunch watch. 

This only happens with dependencies which are ignored by brunch, this is, the files starting with `_`. But this is the quite usual, that's why I've made this request that solves this problem.