Allow parsing multi-line strings in completions.

Mostly needed later for IJulia, as for Jupyter/IPython
and the new v5 message spec the completion send the all code cell
as well as the cursor position, so this methods is almost perfect to be
used, except it does not accept more than one expression before raising.

This fixes that by allowing parsing of multiple line string

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I might be missing something though, so I would appreciate feedback, and any guide on this part of the internals of julia.