feat(processor): add abbreviation support

[lewisacidic]

citeproc-js has abbreviation support built in by supplying additional methods on the sys object: getAbbreviation.

Journal abbreviations are available here. We could extract the relevant fields into JSON and optionally load them in simple-cite so that styles that prefer short form i.e. J. Chem. Inf. Model. can set it even if the item only has the full journal name (i.e. Journal of Chemical Information and Modelling).

• make package "journal-abbreviations" which can autoupdate according to the link, maybe using a scheduled CircleCI workflow.
• support a new option to Processor, abbreviations, which takes a cache object, shaped as so:

exports.default = {
  default: {
    "container-title": {
      "Journal of Chemical Information and Modelling": "J. Chem. Inf. Model.",
      ...
    }
  }
}