feat: add the ability to update a document in the index

[gbittencourt]

Hello all,

I felt the lack of an ability to update a document in the index. I was doing: removeAt(doc, docIdx) add(doc) But at this way I was loosing the docRef. I hope this commit will solve the problem :)

I also added a test to it.

[gbittencourt]

Hey, any maintainer interested in this MR? If there is something not clear enough on my commit, I can fix it, just let me know :)

[krisk]

Pardon the hiatus. Been an interesting past few months.

This is a neat feature. Let me get to it this weekend.

[RealAlphabet]

This feature is definitely needed.

Is fuse.js still maintained? If not, do you know of better alternatives?