Alternative email addresses gathering method

[b4dnewz]

Right now the only supported method for email gathering is through Hunter.io API but this requires to be registered and have a valid API key which has usage limits and fees for non-basic use.

It could be useful to gather emails using search engines (google, bing, ...) looking for common email pattern and extracting from results, for example as it's done in m4ll0k/Infoga package which uses various free techniques to gather as much emails as possible.

This should be the default method used in email gathering task and eventually extended with Hunter.io API for more precise and reliable results.