Allow non-default databases in cloud bucket to be downloaded on first run

[bede]

Currently a custom database can be specified using --index. However, this needs to be already available on a local filesystem. If no index is specified and the default (human-t2t-hla) is not cached locally, it is downloaded. It would be useful if applications depending on Hostile could override the default database such that a custom database could be automatically downloaded on first run of not already present.

Another way to do this would be to implement a database / fetch subcommand

[bede]

Use environment variable to override default DB?

[bede]

Leaning towards --index accepting either an index path OR a name of a standard index in the cloud bucket, if not already cached locally. An alternative is to have separate args (e.g. --index and --custom-index for standard indexes referenced by name, or custom indexes referenced by path, respectively)

[bede]

Complete, to be released in 1.0.0 shortly