Fix handling URLs where username is not at the end of the path

[louisbourque]

Fixes #93

This PR fixes handling clicking the Follow button while on a URL that doesn't end with the user's username.

For example: https://mastodon.url/@username/with_replies

Instead of always using the 0th index, we search for the first occurrence of a path section that start with @, indicating this is the username.

[rugk]

Hi @louisbourque,
first of all, thanks for your first contribution to this project! 🎉 👍 🏅
I hope you'll like this project and enjoy hacking on it… 😃

BTW: Next time, try to avoid doing a pull request from the master branch, because you can run into problems when you have a "non-clean" master that does not follow this repo here (i.e. "upstream"). See this article for details. Anyway, this is only a tip for the next time. 😃

[rugk]

The changes look good. Do you also want to add yourself to the CONTRIBUTORS.md file?

[louisbourque]

Ah, interesting. I didn't realize the source branch name would make a difference. I don't do many PRs from forked repos. Thanks for the heads up.

Thanks for the offer, but I'm happy with only being in the git history. If I contribute more I'll add myself.