National independence day goodie

**What does your Instant Answer do?**
Answers what the independence day of a country is.

**What problem does your Instant Answer solve?**
It answers quite a specific question that has one right answer.

**What is the data source for your Instant Answer?**
Wikipedia, list of national independence days 
https://en.wikipedia.org/wiki/List_of_national_independence_days

**Why did you choose this data source?**
It was suggested on the ddg forums 
https://duck.co/ideas/idea/4546/national-independence-day

**Are there any other alternative (better) data sources?**
Perhaps?

**What are some example queries that trigger this Instant Answer?**
- *independence day, papua new guinea*
- *when is the independence day of norway?*

**Which communities will this Instant Answer be especially useful for?**
The general population

**Is this Instant Answer connected to a DuckDuckHack [Instant Answer idea](https://duck.co/ideas)?**
Yes, https://duck.co/ideas/idea/4546/national-independence-day


**Which existing Instant Answers will this one supercede/overlap with?**
None that I know of.

**Are you having any problems? Do you need our help with anything?**
Not any in particular. Is there a perferred way to store data in text file? I currently use JSON.

**Where did you hear about DuckDuckHack?**
Via the learn more button on the front page of duckduckgo.com, I think. 

**What does the Instant Answer look like?**
Returns a plain text string.

## Checklist
Please place an 'X' where appropriate.

```
[X] Added metadata and attribution information (https://duck.co/duckduckhack/metadata)
[X] Wrote test file and added to t/ directory (https://duck.co/duckduckhack/goodie_tests)
[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
    Developed with firefox on ubuntu, but I assume this doesn't apply since is a plaintext answer
    
```

I think we're hoping for an awful lot to hit this exact string.  I'm not sure how, exactly, but it would be cool to have some aliases for some of the more complex ones.