Add an .svg public method to the API

[lleaff]

This method does the exact same as .image when used to output an SVG, but it outputs the raw Buffer from Graphviz instead of writing it to a file and returning the file path.

For a project where I use madge to generate many images, bypassing the file system improves performance and simplifies the logic a lot.

Obviously this is just a proposition, the method name could be changed and I could add better tests.

[pahen]

Thanks for the contribution! It's a good suggestion but it would be nice with a more generic solution which is not restricted to generating a SVG. What about adding a .buffer method instead?

const madge = require('madge');

madge('path/to/app.js', {type: 'svg'})
  .then((res) => res.buffer())
  .then((output) => {
    console.log(output.toString());
});