allow metasploit utils paths to be set by the user + minor changes

Except in specialized distributions like `Back|Track`/`Kali`, metasploit utils are often not in the `PATH`, current implementation requires them to be, without any alternative. This fixes it by storing `msfpayload`, `msfencode` and `msfcli` paths as variables.

Other changes included in this pull request are purely self-opinion-led cosmetic improvements. Still hope you'll like them.