feat: add custom label support via *_label config options

[gbomacfly]

feat: add custom label support via *_label config options

Closes #33

What does this PR do?

Adds optional *_label config options that allow users to override the default label text for each header field.

Changes

• config.lua: new *_label options added to defaults, new get_label() helper function
• core.lua: all header fields now use get_label() instead of constants directly
• README.md: new section documenting the *_label options, defaults table updated
• tests/header/setup_spec.lua: new label defaults added to expected config
• tests/header/add_headers_spec.lua: new test case for custom labels

Usage

require("header").setup({
    file_name_label = "File:",
    author_label = "Author:",
    project_label = "Project:",
    date_created_label = "Created:",
    date_modified_label = "Modified:",
})

Notes

• Default is nil for all *_label options → fully backwards compatible, no breaking change
• When nil, the built-in label from constants is used as before

[gbomacfly]

First time contributing via PRs — I think it's working now ;)

[attilarepka]

Thanks!