Add support for Finnish

Tradukisto is the library what converts numbers to human-readable words - for example from `14` to `fourteen`. This issue was created to add support for counting in new language. Depending on how numeral system in your language is similar to other languages (english, arabic, german, slavic) and how many exceptions it have - it can be easy or more demanding.

Implementing [unit tests](https://github.com/allegro/tradukisto/blob/master/src/test/groovy/pl/allegro/finance/tradukisto/internal/languages/english/EnglishValuesTest.groovy.) will be really helpful for you, I encourage you to create them first (TDD paradigm), before developing new changes in the code. You can run them in terminal with `./gradlew test` or green button in Intellij code editor.

New language needs to have implementation for [Container](https://github.com/allegro/tradukisto/blob/54e45ee94e13aae69aa64e01d934425c8bd0ed01/src/main/java/pl/allegro/finance/tradukisto/internal/Container.java#L119). Depending on how your language works, having conjugation, gender forms, custom chunking (typically divider is for 3 numbers) etc it may be needed to define more advanced container with custom implementations for counting, for example [turkish](https://github.com/allegro/tradukisto/blob/master/src/main/java/pl/allegro/finance/tradukisto/internal/Container.java#L178). Tests will really be helpful for you to see if it needs custom implementations or not.

Also define `Values` - [translations for base numers](https://github.com/allegro/tradukisto/blob/54e45ee94e13aae69aa64e01d934425c8bd0ed01/src/main/java/pl/allegro/finance/tradukisto/internal/languages/english/EnglishValues.java#L17) and for [plural forms (thousands, millions etc)](https://github.com/allegro/tradukisto/blob/54e45ee94e13aae69aa64e01d934425c8bd0ed01/src/main/java/pl/allegro/finance/tradukisto/internal/languages/english/EnglishValues.java#L60).
Feel free to ask if you have more questions, we will try to help you.
https://github.com/allegro/tradukisto/wiki/How-to-start-contributing