Allow user to customize items

[DonnieWest]

Continues from #2710

Allows the user to set their own completion fields

Relies on #2845

[DonnieWest]

@w0rp looks like it's failing on vint although the syntax is correct. Any advice for making it pass?

[w0rp]

The error message printed says there's no scriptencoding, so add one. It's complaining because there a Unicode characters in the file without declaring the script as using UTF-8.

[w0rp]

Just need some tests and a few changes.

[w0rp]

These tests should still pass, as we should keep the existing defaults.

[DonnieWest]

Do we want to maintain strict backward compatibility?

Seems to me we'd want to make these roughly match the kinds that vim has built in:

	v	variable
	f	function or method
	m	member of a struct or class
	t	typedef
	d	#define or macro

In particular, since this is a property it would probably map best to m (not v as it is right now, sorry). Same as the Rust types above. They probably map best to t, right?

I'm happy to maintain compat though or try this new logical mapping

[DonnieWest]

@w0rp sorry to bug you, but still waiting for a definitive response here :)

[DonnieWest]

Sorry for the noise. Still having trouble getting vint to pass on both this PR and #2849

I'm not sure what I'm missing here :/

[DonnieWest]

@w0rp got the linter passing :)

Once I get feedback on the completion items, I should be good to go

[DonnieWest]

AppVeyor issue appears to be caused by html-beautifier as noted in the other PR

[DonnieWest]

@w0rp looks like the html-beautifier fix also fixed things over here. Just waiting for feedback on whether we want to actually retain backwards compat or if we're fine changing some types returned by default during completion.

Thanks :)

[w0rp]

I think this is good as it is. Let's go with this.

[w0rp]

Cheers! 🍻

[DonnieWest]

Thanks!