Use SVG badges / Fix .npmignore

* I updated the image format of the badges in README from PNG to SVG. SVG badges look beautiful on retina displays.
* `node_modules` directory is ignored by default. According to the npm developer guide, [*Keeping files out of your package*](https://www.npmjs.org/doc/misc/npm-developers.html#Keeping-files-out-of-your-package):

  > Additionally, everything in `node_modules` is ignored, except for bundled dependencies. npm automatically handles this for you, so don't bother adding `node_modules` to `.npmignore`.