Default main.cljs.edn generation should be customizable from other tasks

The need came out from a `boot-cljs-devtools` [issue](https://github.com/jupl/boot-cljs-devtools/issues/5).

The scenario: the user does not specify `cljs.edn` files explicitly. In general a task before `cljs` might need to add a `:require` or `:init-fns` clause to the default `main.cljs.edn` generated by `cljs`.

At the moment there is no way to do that and `cljs-devtools` blindly ignores the fact that we have no `.cljs.edn` skipping adding stuff to it. Of course, changing the order works (e.g.: `(boot ... (cljs) (cljs-devtools)`, but in general my idea is to have ways to add stuff to `:require` (or `:init-fns` or ...) from tasks before `cljs`.

The idiomatic way is to add metadata to files in the fileset but I wanted to ask here what we should mark and of course if you folks consider this a good idea or total garbage :smile:

Thanks!