Don't add the link if it already exists

[Efreak]

When I use browser history to to back/forward on github, every time I see the main repo page, it creates a new link below the others. This fixes that--it switches to using an id instead of a class, and it'll only make an element if there isn't one already.

I'm using chrome with tampermonkey if that makes a difference.

[jerone]

Thanks for this great PR. I encountered this issue many times myself (double links), but never got around to fix it.

I gave some minor style feedback; if you fix those I'll merge and release as soon as I can.

[jerone]

Awesome thanks! :tada: :ship: