Added geometry formula shower / calculator

I excuse me for my errors in English
**What does your instant answer do?**
This goodie shows geometric formulas (e.g. circle area) or calculates them if sizes are specified.

**What is the data source for your instant answer? (Provide a link if possible)**
The german "Tafelwerk", because it is a good formulary

**What are some example queries that trigger this instant answer?**
formula circle area
for more: see test file

**Which communities will this instant answer be especially useful for? (gamers, book lovers, etc)**
people without a formulary

**Is this instant answer connected to a DuckDuckHack [instant answer idea](https://duck.co/ideas)?**
Yes, https://duck.co/ideas/idea/1053/geometry-calculator.

**Which existing instant answers will this one supercede/overlap with?**
Not known yet

**Are you having any problems? Do you need our help with anything?**
Not yet

```
[x] Added metadata and attribution information
[x] Wrote test file and added to t/ directory
[] Verified that instant answer adheres to design guidelines(https://github.com/duckduckgo/duckduckgo-documentation/blob/master/duckduckhack/styleguides/design_styleguide.md)
[] Tested cross-browser compatability
[x] Tested only on Linux 64-bit Firefox
```