implemented 'experimental' switch

doc/advopt.txt

@@ -91,4 +91,5 @@ Advanced options:
   --verbosity:0|1|2|3       set Nim's verbosity level (1 is default)
   --cs:none|partial         set case sensitivity level (default: none);
                             do not use! this setting affects the whole language
+  --experimental            enable experimental language features
   -v, --version             show detailed version information

doc/manual/pragmas.txt

@@ -495,3 +495,21 @@ identifier that can be used to enable or disable it:
 This is often better than disabling all warnings at once.
 
 
+experimental pragma
+-------------------
+
+The ``experimental`` pragma enables experimental language features. Depending
+on the concrete feature this means that the feature is either considered
+too unstable for an otherwise stable release or that the future of the feature
+is uncertain (it may be removed any time).
+
+Example:
+
+.. code-block:: nim
+  {.experimental.}
+
+  proc useUsing(dest: var string) =
+    using dest
+    add "foo"
+    add "bar"
+

doc/manual/stmts.txt

@@ -488,7 +488,8 @@ Instead of:
 Using statement
 ---------------
 
-**Warning**: The ``using`` statement is highly experimental!
+**Warning**: The ``using`` statement is highly experimental and has to be
+explicitly enabled with the `experimental`:idx: pragma or command line option!
 
 The using statement provides syntactic convenience for procs that
 heavily use a single contextual parameter. When applied to a variable or a