[Feature Request] Use the system cache directory, instead of a local directory

[Hate9]

Glowpub currently uses ./cache for storing cache data, but realistically, it should use a subfolder of the operating system's preferred cache directory.

In a similar project of my own, I used dirs' cache_dir function for this.

Possible concerns: This change would be some degree of a breaking change, which could theoretically upset someone's workflow.
Possible upsides: If we end up making a breaking change to the defaults (and presumably doing a major version bump to signify it), we could change all the other defaults to more sane options too.

[QuartzLibrary]

Using an inline cache was intentional on my part because I wanted to be able to check it into git (useful for archival and diffing).

Diffing was especially useful during development, so it seems fine and often desirable to use the system cache folder.

Not sure whether it should be the default though, as I don't know what other people's workflows are like.
For example, an author could keep a versioned archive of their writing. I don't know that's the case, but a single such case would be sufficient for me to say 'not worth it' to changing the default.

In the meantime, as a workaround, sym-linking the inline folder to the system cache should work.

CC: https://github.com/dirs-dev/directories-rs (A slightly higher level crate from the same dev of https://github.com/dirs-dev/dirs-rs)