[FEATURE] Command-line interface (CLI) and Read-Eval-Print loop (REPL)

[tommens]

Describe the new functionality you would like to see

• In order to make the Calculator usable in practice, a command-line interface (CLI) should be implemented to introduce and evaluate arithmetic expressions from the console or terminal. To do so, we need support for a a Read-Eval-Print Loop (REPL): when running the application, the user gets a command-line prompt on his console, where he can enter an arbitrary string. This string will be read, parsed and converted into a syntactically valid expression, or a syntax error will be returned if the string does not respect the correct syntax. Valid expressions will be evaluated, and the result of the evaluation will be printed on the console. Then, we can continue with entering a new string to repeat the same process over and over again.
• The CLI should come with a built-in help function with instructions on how to use the different functionalities of the calculator.

Prerequisite
A prerequisite for realising this feature is the presence of an expression parser, as described in issue #8 .