Generate default theme without initializing

[Kinrany]

Currently it's possible to generate theme files as part of mdbook init, but not after that.

It would be convenient to be able to generate all of the files that weren't changed, tweak the ones you want to change, and delete the rest (to keep the number of files small).

[joshrotenberg]

Does this warrant an additional subcommand (i.e. mdbook theme)? Or possibly an additional flag to init like --theme-only?

[Kinrany]

Third, weird option: detect that the project already exists, ask the user if they want just the theme.

Probably bad for discoverability.