Simplify command line usage through default parameter

[ptrstn]

Currently, if one wants to translate text via the command line, either the parameter -t for text or -f for file must be provided.

The general case is probably that a text rather than a file is to be translated. Therefore the parameter -t is a suitable default parameter.

This could ease up the usage to look like this:

deepl spanish russian "¡Buenos días!"

Maybe even like this (without quotes):

deepl spanish russian ¡Buenos días!

However, this would assume that the words are space separated. This behavior should be double-checked and given a second thought.