Allow translator lazy-loading

[cprecioso]

Currently, a preval-ed file gives Zotero access to all the translator files, directly loaded at launch. It'd be great to allow Zotero to request and load them when needed, instead of eagerly.

[cprecioso]

Ideas:

• Bring translators.js from translation-server into this module, and rewrite it to use requires instead of the fs module. This would mean managing more code in this project, which is originally intended as a thin shim on top of translation-server.

• Write an accessor for the code property of the returned translator JSON, that dynamically requires the code of the translator. This means less work, but might not be as compatible or effective.