Refactored preproc functions

• extract_key_expressions is now faster by directly utilizing library(stringi) instead of library(stringr)
• Added function prepare_translation_table (based on {data.table} which is called by save_to_json and save_to_csv. It allows to read-in previously existing translation files (json or csv) and updates them without overwriting existing translations (implemented to realize create_translation_file's update parameter)
• create_translation_file is now able to search through multiple R scripts. Furthermore, It gained the parameters key_language, translated_languages and update. The languages are reflected in the resulting translation file with placeholder ""
• amended tests