Allow transform modifiers in templates

[bdarcus]

Modify:

• citar-display-transform-functions so it becomes a list of shortcuts to include in template specs (like "${author:20%etal}" where etal is the key that then points to a list of a function symbol and optional arguments).

• citar-get-display-value so it allows an optional transform function

• citar-format--parse to include transform list

• formatting functions to properly integrate the above

Close #694

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This is an alternative to #734; I think easier to implement and configure, and more flexible.

I think it all works, and unless I hear otherwise, I'll likely merge it tomorrow, March 13.

EDIT: I got sidetracked by a stupid variable declaration bug; now fixed.

@andersjohansson @pprevos - any feedback on this? I think you were, or may be, interested in such a feature.

PS - It has occurred to me that perhaps when strings are transformed, the original value should still be included as hidden (making use of display vs hidden properties)?

(propertize "Doe, J and Jones, K and Smith, L" 'hidden t 'display "Doe et al.")

[pprevos]

Hi Bruce, good stuff.

There is a package for org-ref that prettifies citations. Is that what you meant by hidden?

[bdarcus]

There is a package for org-ref that prettifies citations. Is that what you meant by hidden?

No, that's what org-cite-csl-activate does.

More what we already do a bit of in the minibuffer candidates. But it's an optimization to consider later.

[andersjohansson]

Seems like a great idea! I dug into the parsing and realized this: #743