Ergonomics and docs

[fbenkstein]

This PR tries to attempt a couple of issues at once:

• Add a SquidBuilder struct as an alternative to Options. I believe this
  makes the code more readable. For example instead of

  let sqids = Sqids::new(Some(Options::new(
    None,
    Some(10),
    None,
  )))?;
  

  We can now write:

  let sqids = Sqids::builder()
    .min_length(10)
    .build()?;
  

  If this is accepted I suggest deprecating Options in favor of
  SqidsBuilder and removing it in the 1.0 release.
• Pull in the README.md file as a crate-level doc-comment. This makes the
  docs much more approachable since now the main doc page has actual useful
  information instead of being a barren waste land. Since the README.md
  links to the LICENSE file I used a small hack to ensure that this link is
  still valid.
• Pulling in the README.md file automatically turned the code examples into
  documentation tests. These were immediately broken because of missing imports
  and error handling. I've added the necessary boilerplate. It's hidden by
  rustdoc but unfortunately looks a bit ugly when viewing the README.md on
  Github.
• Add basic doc comments to every public item and warn when new items are added
  without docs. If I were maintainer of this crate, I'd turn this into a
  deny.

Let me know if you want me to pull this apart into multiple PRs instead.

[4kimov]

@fbenkstein This PR is pure 🔥 -- thank you. Merged 💪

I've sent you an invite with maintainer rights to this repo, so you can push without being blocked by me.

Some follow-ups:

• The builder pattern is great, I've been eyeing it for a while. I don't mind removing the Options even in the next release (0.4.0); we're going to be pre-production for a while since we're testing not just the individual implementations, but the algo/spec as well.

• All the readme/docs updates are great 👍

• If I were maintainer of this crate, I'd turn this into a deny

  That's cool with me.

[fbenkstein]

@4kimov thanks for the quick turnaround.

I've sent you an invite with maintainer rights to this repo, so you can push
without being blocked by me.

Thank you, accepted. I'll send a PR anyway unless it's something really tiny.