Style changes (code and comments)

My first pull request ever, so bear with me please. I fixed some inconsistencies in the comments (minor). Furthermore, I also changed the creation of the <symbol>-elements from a string-based approach to a more object-oriented approach using cheerio. I hope the latter improves readability and maintainability; it does that for me at least.

Thanks for making this tool, I am really excited to see SVG is used more and more on the web!