Introduce German language support

As a native German speaker, I would like to contribute german support. The translation corresponds to the official [gherkin i18n](https://github.com/cucumber/gherkin/blob/master/lib/gherkin/i18n.json#L128).

Translations literally (for reference)

<table>
<tr><td>Funktionalität</td><td>functionality</td></tr>
<tr><td>Aspekt</td><td>aspect</td></tr>
<tr><td>Anwendungsfall</td><td>usecase</td></tr>
<tr><td>Geschehnis</td><td>happening</td></tr>
<tr><td>Beispiele</td><td>examples</td></tr>
<tr><td>Offen</td><td>open</td></tr>
<tr><td>Einzig</td><td>as the only one</td></tr>
<tr><td>Grundlage</td><td>fundamental / foundation</td></tr>
<tr><td>Hintergrund</td><td>background (more like in a canvas-layer sense)</td></tr>
<tr><td>Vorausgesetzt</td><td>assuming that ...</td></tr>
<tr><td>Angenommen</td><td>assuming</td></tr>
<tr><td>Mit, und, aber, außer</td><td>with, and, but, except</td></tr>
<tr><td>Wenn, falls</td><td>if</td></tr>
<tr><td>Dann</td><td>then</td></tr>
<tr><td>Folglich</td><td>followingly</td></tr>
</table>