Refs #254 - Passed output path ignored when fetching subscriptions for...

Passed output path is not ignored anymore (fixed earlier). However there was a confusion about -a flag usage. It was created to simplify merging with the default subscriptions file (adblockpluscore/data/subscriptions.json), but if calling with npm exec the hardcoded path "data/subscriptions.json" becomes relative to the current dir and thus invalid.

In order to make it working we need to know the exact "adblockpluscore" module path to make the path absolute and we can't know it as it's up to Node.

So eventually it does not simplify the webext dev life unfortunately. As an "action item" i think we nee just don't mention about this argument in webext-sdk. And imo we should not delete it as it can still be used if working with "core" scripts.

Any other ideas?