Allow more customisation of defaults for tar_* functions

[Moohan]

Prework

• I understand and agree to help guide.
• I understand and agree to contributing guide.
• New features take time and effort to create, and they take even more effort to maintain. So if the purpose of the feature is to resolve a struggle you are encountering personally, please consider first posting a "trouble" or "other" issue so we can discuss your use case and search for existing solutions first.

Proposal

My specific use case is that I use tar_visnetwork(level_separation = 750L, label = c("time", "size")) quite regularly. It would be great if I could set the default level_separation and label somewhere, either in tar_option_set or in _targets.yaml using tar_config_set.

I think generally this is applicable to a lot of functions and there parameters where it would be good to have the ability to set default options for them.

[wlandau]

I added tar_config_set() configurable defaults for label and level_separation. Hopefully that helps.

There are so many arguments that I prefer to handle this sort of thing on a case-by-case basis.