Allow Forced New Lines

[FlorianRappl]

As discussed in #86 there are cases where the REPL may be misused / misunderstood, e.g., if one wants to play the opening brace in a new line instead of the current line.

The origin of the error lies in the fact that the REPL is by-default single line and enter starts the evaluation. There are some techniques to check for a wanted line ending, but we only cover the simplest cases to continue in another line. Once a statement seems to be complete we trigger the evaluation.

In case of for (...) our simple checks do not tell us that something is missing. Actually, since for (...); is valid we cannot make an exception for this one (and similar cases).

The simplest counter measure is to allow forced new line (e.g., via shift enter). That way the user can just state "okay, now I do not want have the evaluation running, but I first want to supply more lines / information".