Add support for FileReader

• Add FileReader support, many thanks to @DineshSolanki and @frnknglrt for issue request and implementation.

Example:

    val imageString = KVar("")
    Kweb(port = 8008) {
        doc.body.new {
            val input = fileInput()
            input.onFileSelect {
                input.retrieveFile {
                    imageString.value = it.base64Content
                }
            }
            img().setAttribute("src", imageString)
        }
    }