Add "filename" to taint summary

[filipeom]

This property denotes a relative path, meaning it is relative to the location of the taint summary.

Here:

    "filename" : "./src/example-0.js",
    "vuln_type": "code-injection",

The directory structure is implied:

.
├── src
│   └── example-0.js
└── taint_summary.json