Add support to determine custom variables

[joanlopez]

Basically, extend scan.CustomTokens but from the command-line interface.

We can do the following:

• Add a flag like: -V / --variable (or even -var) that accepts a format like: "{FOO}:BAR", so {FOO} tokens present in profiles get replaces with "BAR". It should have support to be specified multiple times.
• Add a flag like: -VF /--variables-file(or even-varfile`) that accepts a text file with multiple lines, following the same convention as on the previous point.
• Make gbounty to automatically look if there's any variables.txt file within the specified profiles path, and if so, load it and use it by default.

[joanlopez]

cc/ @wagiro any preference on the CLI flag names? 🤔