SI-4594 Repl settings command

A settings command for the rest of us.

The usual command line options are used, except that boolean flags
are enabled with +flag and disabled with -flag.

```
scala> :settings +deprecation

scala> new BigInt(java.math.BigInteger.TEN) { }
<console>:8: warning: inheritance from class BigInt in package math is deprecated: This class will me made final.
              new BigInt(java.math.BigInteger.TEN) { }
                  ^
res0: BigInt = 10

scala> :settings -deprecation

scala> new BigInt(java.math.BigInteger.TEN) { }
res1: BigInt = 10
```
Multivalue "colon" options can be reset by supplying no values
after the colon.  This behavior is different from the command line.

```
scala> 1 toString
warning: there were 1 feature warning(s); re-run with -feature for details
res0: String = 1

scala> :settings -language:postfixOps

scala> 1 toString
res1: String = 1

scala> :settings
-d = .
-encoding = UTF-8
-explaintypes = false
-language = List(postfixOps)
-nowarn = false

scala> :settings -language:

scala> :settings
-d = .
-encoding = UTF-8
-explaintypes = false
-language = List()
-nowarn = false
```

1 files changed, 26 insertions, 0 deletions

diff --git a/test/files/run/t4594-repl-settings.scala b/test/files/run/t4594-repl-settings.scala
new file mode 100644
index 0000000000..d2335460e5
--- /dev/null
+++ b/test/files/run/t4594-repl-settings.scala
@@ -0,0 +1,26 @@
+
+import scala.tools.partest.SessionTest
+
+// Detected repl transcript paste: ctrl-D to finish.
+object Test extends SessionTest {
+  def session =
+""" |Type in expressions to have them evaluated.
+    |Type :help for more information.
+    |
+    |scala> @deprecated(message="Please don't do that.", since="Time began.") def depp = "john"
+    |depp: String
+    |
+    |scala> def a = depp
+    |warning: there were 1 deprecation warning(s); re-run with -deprecation for details
+    |a: String
+    |
+    |scala> :settings +deprecation
+    |
+    |scala> def b = depp
+    |<console>:8: warning: method depp is deprecated: Please don't do that.
+    |       def b = depp
+    |               ^
+    |b: String
+    |
+    |scala> """
+}