Add remove method

[jharding]

Asked about in #647.

[milotoor]

This would be really useful to have implemented, but I do see the difficulty involved given the structure of the search index. You can't just pop something out of the index's "datums" array without updating the ids of, in the worst case, virtually every single node in the trie. Given that, I'm inclined to agree with @addremove's suggestion (see #946) that an associative array might be a better avenue for organizing the data.

Why not do this:

• convert the datums array into an object
• modify this line, computing the id of the datum with some hashing function
• add the datum into the datums object using its computed id

Of course anywhere else in the search index that leverages datum ids would have to be modified as well.

That would skirt the issue of having ids vulnerable to removal, as there is now no need to completely traverse the trie upon updates to the data. I'm happy to take a stab at implementing this, but your input would be very helpful, @jharding.