REPL :edit command

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It would be nice if the repl could recall symbols and history for editing, either with an external editor or just to put a line in the current jline buffer.

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Imported From: https://issues.scala-lang.org/browse/SI-7637?orig=1
Reporter: @som-snytt
Affected Versions: 2.11.0-M3
See #1067

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@som-snytt said:
Using an external editor is a workaround for multi-line edit problem in jline.

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@adriaanm said:
scala/scala#2706