Allow passive mode to be run on responses exported from other tools

[tomdottom]

Please describe your feature request:

Currently a passive scan can be performed on a response saved in a .txt file, or a directory of .txt files:

nucliei -passive -target ./responses/

It would useful to be able to run passive mode against responses stored in other supported formats, for example a burp export:

nuclie -passive -target burp-export.xml -input-mode burp

Describe the use case of this feature:

This would be use of use in situations such as:

• Security audits where active scanning & fuzzing are out of scope and request/response data is collected by recording manual interaction with services, ie. when testing critical prod systems
• Automated export and passive scans of assets produced by other tools