add a LaTeX unicode completion plugin

This plugin replaces LaTeX commands with the corresponding unicode symbols. For example, \alpha is completed to α and \in is completed to ∈. This is mostly useful for folks coding in Julia, but can be used with any language that allows unicode identifier names, including Python.

The completions are scraped from the Julia documentation using the fetch_unicode_table.py script. This does not happen automatically to avoid downloading random stuff from the internet during build (AFAIK distros are allergic to this), so the table will need to be manually updated from time to time.

An MIT-licensed, header-only trie implementation is used for efficient prefix search. (I tried having this as a git submodule, but the push got rejected. Copying the entire repo into the project is the recommended way by upstream, it doesn't have an install directive).

Not sure if LaTeX completion is the most descriptive name, I'm open to suggestions here.