Add print styling

[Tomassie91]

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Thanks for writing this great plugin! It's great for previewing markdown files. I also use it to convert markdown to PDF, using the print functionality of my browser. Unfortunately, there aren't any print styles in the themes. This adds a border to the GitHub theme, as Huangjj27 noted in #36. It also creates awkward page-breaks, which can be really annoying.

I would greatly appreciate any improvement in this area.

[simov]

Thanks for the feedback @Tomassie91 it's definitely on my TODO list.

[simov]

I added some print styling in v5.1 that should help a little bit with that. It's a work in progress though so let me know if you find anything missing.