Add color support (configurable) for tags highlighting

[yousefakbar]

Add color support (configurable) for tags highlighting

Example:

Add some configuration options like this:

return {
    "yousefakbar/notmuch.nvim",
    opts = {
        notmuch_db_path = "/home/xxx/Documents/Mail",
        maildir_sync_cmd = "mbsync personal",
        keymaps = {
            sendmail = "<C-g><C-g>",
        },

        -- ADDED CONFIGURATION HERE --
        tags = {
          colors = {
            { tag = 'inbox', color = 'red' },
            { tag = 'unread', color = 'cyan' },
          },
        },
    },
}

Then, in notmuch-threads and mail filetypes, display those tags with their corresponding colors

Changes would be in config.lua, and syntax/{notmuch-threads,mail}.vim (optionally syntax/notmuch-hello.vim)