[feature] runningsPath alternative by using a function

[dawadam]

Hi, great job.
It will be practical to use a function for placing header and footer, programmatically, out of a file.
Like this :

        let headerContent = function (pageNum, numPages) {
            return "<h1>Header <span style='float:right'>" + pageNum + " / " + numPages + "</span></h1>"
        }

        let options = {
            cssPath: this.rootPath + "/style.css",
            header: {
                height: "1cm",
                contents: headerContent
            }
        }

        fs.createReadStream(this.rootPath + "/in.md")
            .pipe(markdownpdf(options))
            .pipe(fs.createWriteStream(this.rootPath + "/out.pdf"))

It will be also useful for making custom header and footer be adding variables.