feat WIP: checkpoint initial efforts on stream extraction

Adds ! operator and !! expression, with Python code gen and
  an item in tour.
  Still needs: other languages, check generated docs, and addition
  of standard input to test scheme.

tests/basic.spt

@@ -294,3 +294,36 @@ test emit_several_default [[
 ]] parse succeeds
 /** writes
 3399677527121313*/
+
+/** md
+### Streams are bidirectional
+
+So far we have only sent items to a stream. But we can extract them from
+streams as well, with the `!` postfix operator. `!!` all by itself abbreviates
+`stream!`, i.e., extraction from the standard stream. For example,
+
+```fostr
+**/
+
+/** md */ test custom_hw [[
+<<< "What is your name?\n"
+<<< 'Hello, ' ++ !!
+]] /* **/
+parse to TopLevel(Sequence([
+  DefGets(EscString("\"What is your name?\n\"")),
+  DefGets(Concat(LitString("'Hello, '"),DefEmits()))
+]))
+/** accepts
+Kilroy
+**/
+/** writes
+What is your name?
+Hello, Kilroy**/
+
+/** md
+```
+
+queries users for their name and then writes a customized greeting. It also
+illustratesthe use of `++` for string concatenation, as opposed to `+` for
+(numerical) addition.
+**/