Remove `Cargo.lock` from `.gitignore`

[jontze]

First of all, thanks for the good work on this project!

I recently noticed that the repo doesn't include the Cargo.lock file and wondered, if this is on purpose.

To ensure reproducible builds it's recommended to add the Cargo.lock file to version control - beside the project is intended to be used as a lib.

However, I know that some people seem to use this project as a library as well, but the main use case is AFAIK to be used as compiled executable and therefore we would benefit from more reproducible builds due to the lock file.

Refs.:

• Cargo Book FAQs: https://doc.rust-lang.org/cargo/faq.html#why-do-binaries-have-cargolock-in-version-control-but-not-libraries
• Long and old Discussion in an cargo issue: rust-lang/cargo#315 with basically this suggestion: https://github.com/rust-lang/cargo/issues/315#issuecomment-51075730

Feel free to close this PR, if you disagree :smile:

[SichangHe]

For future reference, Cargo.lock has been ignored since #14 when @Matthewacon added a number of katex options.

I cannot see why @Matthewacon did that, and I have no objection to this PR.