Add Plural goodie

**What does your instant answer do?**
Returns the plural form(s) of an English noun.

**What problem does your instant answer solve (Why is it better than organic links)?**
Saves the user from navigating to a dictionary page and searching for the plural form.

**What is the data source for your instant answer? (Provide a link if possible)**
Wiktionary: parsed from [this dump file](http://dumps.wikimedia.org/enwiktionary/20140522/enwiktionary-20140522-pages-articles.xml.bz2)

**Why did you choose this data source?**
It's comprehensive, accurate and free

**Are there any other alternative (better) data sources?**
Not aware of any better source for free.

**What are some example queries that trigger this instant answer?**
"plural starfish"
"how do you pluralize supernova"

**Which communities will this instant answer be especially useful for? (gamers, book lovers, etc)**
Writers, students.

**Is this instant answer connected to a DuckDuckHack [instant answer idea](https://duck.co/ideas)?**
Yes, [this one](https://duck.co/ideas/idea/802/pluralize-english-words)

**Which existing instant answers will this one supercede/overlap with?**
As the user could potentially request the plural form of any noun, it would be possible to simultaneously trigger any other instant answer that triggers on a noun.

**Are you having any problems? Do you need our help with anything?**
- Would this be better as a Fathead? The main data file (plurals.txt) is a hefty 3.4 MB.
- At the moment it's designed to accept Unicode/UTF-8 characters in requests (e.g. 'what is the plural of œuvre'). These work in testing with `duckpan query` but don't seem to trigger when testing in the browser with `duckpan server`. I can't figure out if DDG is filtering out instant answers for non-ASCII queries, or if this is a problem in my code.
- Is there a better way to generate the Wiktionary URLs?

**\*\*Note:** Please attach a screenshot for new instant answer pull requests, and for pull requests which modify the look/design of existing instant answers.

![image](https://cloud.githubusercontent.com/assets/726198/3266074/438b87ec-f2a3-11e3-86ee-06f3dba50cc2.png)


## Checklist
Please place an 'X' where appropriate.

```
[X] Added metadata and attribution information
[X] Wrote test file and added to t/ directory
[Xish] Verified that instant answer adheres to design guidelines(https://github.com/duckduckgo/duckduckgo-documentation/blob/master/duckduckhack/styleguides/design_styleguide.md)
[X] Tested cross-browser compatability

    Please let us know which browsers/devices you've tested on:
    - Windows 8
        [] Google Chrome   
        [] Firefox         
        [] Opera           
        [] IE 10           

    - Windows 7
        [] Google Chrome   
        [] Firefox         
        [] Opera           
        [] IE 8            
        [] IE 9            
        [] IE 10           

    - Windows XP
        [] IE 7            
        [] IE 8            

    - Mac OSX
        [X] Google Chrome   
        [X] Firefox         
        [] Opera           
        [X] Safari          

    - iOS (iPhone)
        [] Safari Mobile   
        [] Google Chrome   
        [] Opera           

    - iOS (iPad)
        [] Safari Mobile   
        [] Google Chrome   
        [] Opera            

    - Android
        [] Firefox         
        [] Native Browser  
        [] Google Chrome   
        [] Opera
```

I will never forget your test cases.