Add /usr/local/etc as possible location for config files

[alexmerkel]

The change make theHarvester search for proxies.yaml etc. in /usr/local/etc/theHarvester/ directory in addition to /etc/theHarvester and the current directory. This allows system-wide configs to be installed without root privileges, e.g. via Homebrew.

Additionally, I refactored the code in core.py at bit. Now there is one method to read the api-keys.yaml file and all service-specific methods just call this function and pass on the right key from the returned object.

[L1ghtn1ng]

@alexmerkel Thanks for this and this is call that you have added homebrew support, the other thing that you will need to look at though is the stash.py for the sqlite DB or does the current way we do it work? it would need to stay that way but like how you have done the api-keys if it needs to be modified

[alexmerkel]

The location of the sqlite database is fine imo. It is written to a user controlled location (~/.local), so installing (and running) theHarvester as a non-root user works. I added the other file locations because the original ones are in a location (/etc) typically not writable for normal users.