Add library crate

[PeterlitsZo]

This PR is based on #193. So do not merge me before when #193 is merged, please.

This PR is going to solve #194. It supports users can:

sd::replace("foo", "bar", sd::ReplaceConf {
    source: sd::Source::with_files(vec!["./foo.md", "./bar.md"]),
    ..sd::ReplaceConf::default(),
})

Maybe using a builder is a great idea? And I also need more tests for it.

Your advice is important to me, @CosmicHorrorDev and @fujidaiti. What do you think?

[fujidaiti]

Looks good, thank you! One thing I'm wondering, though, is it possible to use &str instead of String as the type for find and replace_with? Otherwise, we would always have to create and move Strings. In the current implementation, for example, replace("foo", "bar", ...) is not possible.

[PeterlitsZo]

Of course.

[CosmicHorrorDev]

Since this is building off of #193 I'm going to put this on the backburner milestone as well

[CosmicHorrorDev]

Alright the release is out, so I'm back to revisiting this. I posted more of my updated thoughts here (#194 (comment)), but the gist is that too many things are currently changing and more stuff is likely to change in the future within sd, so I don't think it's a good idea to split out a separate library crate at the moment

Thanks for your contribution nevertheless, and sorry for changing my mind on this 🙇