feat: Add noindex file functionality to skip dirs

[joshbeard]

Implement a new feature that allows skipping directories during indexing when they contain a specified noindex file(s). This provides users with a simple way to exclude private directories, prevent indexing of temporary files, and selectively control which directories appear in indexes.

source
├── four
│   └── foo.txt
├── one
│   └── foo.txt
├── three
│   ├── foo.txt
│   └── .noindex
└── two
    └── foo.txt

web-indexer --source source --target target --recursive --noindex-files .noindex
2025/03/14 23:33:47 INFO Generating index for sample
2025/03/14 23:33:47 INFO Skipping source/three (found noindex file .noindex)
2025/03/14 23:33:47 INFO Generated target/four/index.html
2025/03/14 23:33:47 INFO Generated target/one/index.html
2025/03/14 23:33:47 INFO Generated target/two/index.html
2025/03/14 23:33:47 INFO Generated target/index.html

target
├── four
│   └── index.html
├── index.html
├── one
│   └── index.html
└── two
    └── index.html

Resolves #67