Invoke REPL from code

[som-snytt]

A FAQ is how to invoke REPL from code.

Document the common entry points.

[som-snytt]

This PR experienced an intermediate failure state, I don't know if it's meaningful:

[tut] compiling: ./pt-br/cheatsheets/index.md
[tut] compiling: ./pt-br/tutorials/tour/_posts/2017-02-13-xml-processing.md
Exception in thread "main" java.lang.NullPointerException
	at scala.reflect.internal.SymbolTable.exitingPhase(SymbolTable.scala:256)
	at scala.tools.nsc.interpreter.IMain$Request.x$20$lzycompute(IMain.scala:896)
	at scala.tools.nsc.interpreter.IMain$Request.x$20(IMain.scala:895)
	at scala.tools.nsc.interpreter.IMain$Request.headerPreamble$lzycompute(IMain.scala:895)
at scala.tools.nsc.interpreter.IMain$Request.headerPreamble(IMain.scala:895)

Wait a sec, it had gone green. The failed check prevailed.

[lrytz]

missing interpolator?

[som-snytt]

Yes, that was to show the warning output. It should probably explain expected output.

[lrytz]

sorry, thanks :)

[som-snytt]

The script println is missing.

hello, world
OK!
Sorry, try again.
<console>:13: warning: possible missing interpolator: detected interpolated identifier `$who`
        println("hello, $who")
                ^
error: No warnings can be incurred under -Xfatal-warnings.
Sorry, try again.
Welcome to Scala 2.12.1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_111).
Type in expressions for evaluation. Or try :help.

scala> :quit
Welcome to Scala 2.12.1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_111).
Type in expressions for evaluation. Or try :help.

scala> res0: String = hello, world

scala> :quit
Output is Welcome to Scala 2.12.1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_111).
Type in expressions for evaluation. Or try :help.

scala> hello, world
res0: Int = 42

scala> :quit

[SethTisue]

@som-snytt is this ready for merge?

[SethTisue]

(feel free to use your own merge powers if so)

[som-snytt]

Let our powers merge!

The doc will change soon anyway with Friendly Repl. FREPL. Or EPFL FREPL.

If anyone reads this and complains, I will make amends by making emends.

[SethTisue]

🎵 Just want to drink my REPL wine, down in the Lightning Bar 🎵