Cache function list from finfo data

[valbucci]

In PR #62 it was mentioned:

In the future is may be worth looking into setup_function_list and check if we can avoid running the same command twice when finfo and other extract modes like bytes and func-cfg are invoked together.

This PR does exactly that. If finfo is specified as a data extraction mode, it's processed first, and the data resulting from it is recycled to set up the function list cache. Furthermore, the setup_function_list method checks if a finfo data file already exists in the output directory before running aflj and, if it does, it uses that information to generate the function list.