Initial SourceWithContext, FlowWithContext, FlowWithContextOps.

(based on https://github.com/akka/akka/compare/master...jrudolph:jr/withContextOps)

akka-docs/src/main/paradox/stream/operators/Source/startContextPropagation.md

@@ -0,0 +1,18 @@
+# Source.startContextPropagation
+
+Turns a Source into a SourceWithContext which can propagate a context per element along a stream.
+
+@ref[Source operators](../index.md#source-operators)
+
+@@@div { .group-scala }
+
+## Signature
+
+@@signature [Source.scala](/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala) { #startContextPropagation }
+
+@@@
+
+## Description
+
+Turns a Source into a SourceWithContext which can propagate a context per element along a stream.
+The function passed into startContextPropagation must turn elements into contexts, one context for every element. 

akka-docs/src/main/paradox/stream/operators/index.md

@@ -27,6 +27,7 @@ These built-in sources are available from @scala[`akka.stream.scaladsl.Source`]
 |Source|<a name="range"></a>@ref[range](Source/range.md)|Emit each integer in a range, with an option to take bigger steps than 1.|
 |Source|<a name="repeat"></a>@ref[repeat](Source/repeat.md)|Stream a single object repeatedly|
 |Source|<a name="single"></a>@ref[single](Source/single.md)|Stream a single object|
+|Source|<a name="startcontextpropagation"></a>@ref[startContextPropagation](Source/startContextPropagation.md)|Turns a Source into a SourceWithContext which can propagate a context per element along a stream.|
 |Source|<a name="tick"></a>@ref[tick](Source/tick.md)|A periodical repetition of an arbitrary object.|
 |Source|<a name="unfold"></a>@ref[unfold](Source/unfold.md)|Stream the result of a function as long as it returns a @scala[`Some`] @java[`Optional`].|
 |Source|<a name="unfoldasync"></a>@ref[unfoldAsync](Source/unfoldAsync.md)|Just like `unfold` but the fold function returns a @scala[`Future`] @java[`CompletionStage`].|
@@ -285,6 +286,7 @@ For more background see the @ref[Error Handling in Streams](../stream-error.md)
 @@@ index
 
 * [combine](Source/combine.md)
+* [startContextPropagation](Source/startContextPropagation.md)
 * [fromPublisher](Source/fromPublisher.md)
 * [fromIterator](Source/fromIterator.md)
 * [cycle](Source/cycle.md)