Refactor code and add German negation

[MiriUll]

• used factory pattern to group different languages
• added Negator_DE for German negation
• DERBI (needed for German inflection) is WIP, the dependency should be changed to the original repo once the installation issues are fixed

[dmlls]

Hi @MiriUll. Thanks for the PR!

I have added a commit revamping the project structure so that further negators for new languages can be easily added under negators.

The usage now remains quite simple, e.g.:

from negate import Negator

negator = Negator("de")
negator.negate_sentence("Es sieht gut aus.")

Additional, language-dependent parameters can also be passed (they must be named):

negator.negate_sentence("Ich habe eine Idee.", strategy="kein")

It is still left to do some further code refactoring (since there are a number of repeated methods for both English and German), and write tests for German. In fact, there seem to be a number of bugs, e.g.:

negator.negate_sentence("Ich habe Hunger.")   # ['Ich habe keine Hunger.']
negator.negate_sentence("Es funktioniert!")   # ['Es funktioimmerrt!']

But in order not to make this PR too large, and since this is already a good beginning for German, this can be done in further PRs.

Let me know if everything looks good to you and I will then merge it :)