User-friendly interface

[adelezaini]

This PR implements some functionalities to make the interaction with users more intuitive:

• [x] change input scene filename as positional argument to pass;
• [x] add some output messages during the program execution;
• [x] add possibility to set 'a' and 'gamma' parameters on 'render' mode too;
• [x] set default output filename with creation data and time.