A Grunt Copy process function to embed content of files into other files.

grunt.copy.includeFile.js

/**
 * includeFile() - embeds a file content within another. Meant to be
 * used from the copy task as a 'processContent' function. The following
 * tokens can be used in files: <br>
 *
 *  -   BUILD_INCLUDE('file')
 *  -   /* BUILD_INCLUDE('file') *\x47
 *  -   &lt;!-- BUILD_INCLUDE("file") --&gt;
 *
 * In addition, options can be added to the token above that further
 * process the file being included:
 *
 *  -   BUILD_INCLUDE('file')[option1,option2,option3]
 *
 * Supported options:
 *
 *  -   asJsString : Escapes all double-quotes and new line characters
 *                   in the file
 *
 * @param {String} fileContent
 * @param {String} filePath
 *
 * @return {String} fileContent
 * @see https://www.regexpal.com/?fam=108265
 *
 * @example
 *
 *  ...
 *  copy: {
 *      options: {
 *          expand: true,
 *          process: includeFile
 *      }
 *  }
 *  ...
 *
 */
function includeFile(fileContent, filePath){
    if (fileContent.indexOf("BUILD_INCLUDE") > -1) {
        grunt.log.write("includeFile(): [" + filePath + "] has BUILD_INCLUDE: ");

        // Match:
        //      // BUILD_INCLUDE('file')
        //      /* BUILD_INCLUDE('file') */
        //      <!-- BUILD_INCLUDE("file") -->
        //
        //  Token OPtions:
        //      // BUILD_INCLUDE('file')[options,here,as,array]
        //
        //      asJsString
        //
        var re = /(?:(?:\/\/)|(?:<\!\-\-)|(?:\/\*))[ \t]*BUILD_INCLUDE\(['"](.*)['"]\)(?:\[(.*)\])?(?:(?:[ \t]*\-\->)|(?:[ \t]*\*\/))?/i,
            match, file, fileIncludeOptions;

        while ((match = re.exec(fileContent)) !== null) {

            grunt.log.write(".");
            grunt.verbose.writeln("    Match array: " + match );

            file = grunt.template.process( match[1] );

            grunt.verbose.writeln("    File to embed: " + file );

            file = grunt.file.read( file );

            // If options were set, then parse them
            if (match[2]) {

                fileIncludeOptions = match[2].split(',');

                // If option: asJsString
                if (
                    fileIncludeOptions.some(function(option){
                        return String(option).toLowerCase() === "asjsstring";
                    })
                ) {
                    file = file
                            .replace(/"/g, '\\"')
                            .replace(/'/g, "\\'")
                            .replace(/\r\n|\n/g, "\\n");
                }
            }
            fileContent = fileContent.replace(match[0], function(){ return file; });
        }
        grunt.log.writeln("");
        return fileContent;
    }
    return fileContent;
}

I wrote a plugin that his much more extensive and can be found npmjs.com named grunt-build-include also on GitHub