add --logfile tag and dynamic directory switching to call ssrfmap.py from anywhere

[Ne0re0]

• This PR adds a --logfile flag to define a specific log file. If --logfile is not present, it defaults to SSRFmap.log next to ssrfmap.py, as before.
• It also displays the banner before anything else, so the banner is shown even when using --help or in case of errors.
• Finally, it converts relative file paths to absolute paths to allow switching directories, because the script needs to be executed from the SSRFmap directory to load all modules correctly, and, obviously it switches working directory to launch ssrfmap.py from anywhere avoiding errors.