Added a new Goodie instant answer which converts a word from plural to singular

**What does your instant answer do?**
It converts a word from plural form to singular

**What problem does your instant answer solve (Why is it better than organic links)?**
For a quick answer, it is much faster than navigating to organic links and reading through paragraphs of text for one word.

**What is the data source for your instant answer? (Provide a link if possible)**
None. I am using a CPAN module to do the transformation

**Why did you choose this data source?**
N/A

**Are there any other alternative (better) data sources?**
N/A

**What are some example queries that trigger this instant answer?**
singular tablets

**Which communities will this instant answer be especially useful for? (gamers, book lovers, etc)**
ESL users and primary school students

**Is this instant answer connected to a DuckDuckHack [instant answer idea](https://duck.co/ideas)?**
Not that I am aware of.

**Which existing instant answers will this one supercede/overlap with?**
I did not find any other Goodie instant answer which overlaps

**Are you having any problems? Do you need our help with anything?**
No

**\*\*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/4030646/3794464/77b3bfe8-1bad-11e4-98f3-b84450d1d6bf.png)



## Checklist
Please place an 'X' where appropriate.

```
[X] Added metadata and attribution information
[X] Wrote test file and added to t/ directory
[X] 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

    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
        [] Google Chrome   
        [] Firefox         
        [] Opera           
        [] Safari          

    - iOS (iPhone)
        [] Safari Mobile   
        [] Google Chrome   
        [] Opera           

    - iOS (iPad)
        [] Safari Mobile   
        [] Google Chrome   
        [] Opera            

    - Android
        [] Firefox         
        [] Native Browser  
        [] Google Chrome   
        [] Opera
```
It seems like the Lingua docs suggest you use the lowercase version of the input, we should probably ensure we do that when converting and checking the word