Add Jargon File plugin

# Fathead Pull Request

**What does your instant answer do?**
Shows definitions from the Jargon File.


**What problem does your instant answer solve (Why is it better than organic links)?**
It gives information on some rare terms at a glance.


**What is the data source for your instant answer? (Provide a link if possible)**
http://catb.org/~esr/jargon/jargsrc.tar.gz


**Why did you choose this data source?**
It is the most up-to-date version of the Jargon File.


**Are there any other alternative (better) data sources?**
No. ESR is the "official" (as much as anyone can be) maintainer of the file.


**What are some example queries that trigger this instant answer?**
`considered harmful`, `FUBAR`, `misfeature`, `PEBCAK`, `tarball`


**Which communities will this instant answer be especially useful for? (gamers, book lovers, etc)**
Computer users.


**Is this instant answer connected to a DuckDuckHack [instant answer idea](https://duck.co/ideas)?**
No.


**Which existing instant answers will this one supercede/overlap with?**
None known. Probably Wikipedia for some terms.


**Are you having any problems? Do you need our help with anything?**
No.

## Checklist
Please place an 'X' where appropriate.

```
[X] Added metadata and attribution information
[] Wrote test file and added to t/ directory
[] Verified that instant answer adheres to design guidelines (https://duck.co/duckduckhack/design_styleguide)
[X] Verified that instant answer adheres to code styleguide (https://duck.co/duckduckhack/code_styleguide)
[] Tested cross-browser compatibility
```