Allow for text to follow tokens in pretty-printing

It's weird that it counts against the line length though, like if you
were going to break you could ignore it right? At least, for the
grammar I'm working here....

tests/test_wadler.py

@@ -57,7 +57,7 @@ class JsonGrammar(Grammar):
     LCURLY = Terminal("{")
     RCURLY = Terminal("}")
     COMMA = Terminal(",")
-    COLON = Terminal(":")
+    COLON = Terminal(":", format_follow=" ")
     LSQUARE = Terminal("[")
     RSQUARE = Terminal("]")
     TRUE = Terminal("true")
@@ -164,8 +164,8 @@ def test_layout_basic():
         result
         == """
 {
- "a":true,
- "b":[
+ "a": true,
+ "b": [
   1,
   2,
   3