feat: custom headers

[ArnaudLigny]

This feature provide a new configuration structure to define custom HTTP headers, used by the local preview server.

It's useful to test custom Content Security or Cache Policy.

Structure:

headers:
  - source: <path prefixed with a slash>
    headers:
      - key: <key>
        value: "<value>"

Example:

headers:
  - source: /*
    headers:
      - key: X-Frame-Options
        value: "SAMEORIGIN"
      - key: X-XSS-Protection
        value: "1; mode=block"
      - key: X-Content-Type-Options
        value: "nosniff"
      - key: Content-Security-Policy
        value: "default-src 'self'; object-src 'self'; img-src 'self'"
      - key: Strict-Transport-Security
        value: "max-age=31536000; includeSubDomains; preload"
  - source: /images/*
    headers:
      - key: Cache-Control
        value: "public, max-age=31536000"
  - source: /assets/*
    headers:
      - key: Cache-Control
        value: "public, max-age=31536000"
  - source: /test.html
    headers:
      - key: Foo
        value: "bar"