Exclude files from build when not in spine

[breezykermo]

Fixes #78.

The way I've implemented this means that HTML spines can now be specified as a way to exclude certain files from the sitemap. (Note that title is now optional in spines in general, though it is still required for EPUB spines.)

[html.spine]
vertebrae = ["!draft.*.typ"]

[breezykermo]

I suppose this requires a bump to the manifest version, too. Do you have any experience for a process for this @willcrichton? I've heard good things about cargo-semver-checks, but haven't used it myself anywhere.

[willcrichton]

There's a semver crate which provides routines for checking semver compatibility: https://docs.rs/semver/latest/semver/

[breezykermo]

After some thought, we've decided to version the manifest in step with the crate version for the time being so that it doesn't slow development velocity.