FR: Add more granular optimization options for `tt update`

[tingerrr]

Description

Allow setting the optimization level of the optimized references in tt update to something other than on (full optimization) or off (none at all). Perhaps by adding simple levels from 0 to 5, possibly what oxipng itself offers.

Alternative Solutions

Alternatively, the optimization could be fully configurable (i.e. by deserializing something into oxipng::Options from the config or CLI). Although at this point it may be more future-proof to just let the user run tt update --no-optimize-refs and have them optimize the refs themselves. This could be accomplished by adding hooks, but requires more user setup. I'm unsure if this would hold its weight.

Additional Context

See #219 for why this may be important, some update runs can be 20x as slow as a regular tt run in which case the trade-off might be desirable.