allow option-object in toString to insert namespace and style

[mw75]

solves #36 in a generic way

const css=[
    '.railroad-diagram {',
    '    background-color: hsl(30,20%,95%);',
    '}',
    '.railroad-diagram path {',
    '    stroke-width: 3;',
    '    stroke: black;',
    '    fill: rgba(0,0,0,0);',
    '}',
    '.railroad-diagram text {',
    '    font: bold 14px monospace;',
    '    text-anchor: middle;',
    '}',
    '.railroad-diagram text.label {',
    '    text-anchor: start;',
    '}',
    '.railroad-diagram text.comment {',
    '    font: italic 12px monospace;',
    '}',
    '.railroad-diagram rect {',
    '    stroke-width: 3;',
    '    stroke: black;',
    '    fill: hsl(120,100%,90%);',
    '}'].join("\n");
const svgattr='xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"';
const standaloneSVG=Diagram().format().toString({css,svgattr});```