implement strange setting from FastWriter

include/json/writer.h

@@ -74,6 +74,24 @@ public:
         Default: "\t"
     */
     void setIndentation(std::string indentation);
+    /** \brief Drop the "null" string from the writer's output for nullValues.
+    * Strictly speaking, this is not valid JSON. But when the output is being
+    * fed to a browser's Javascript, it makes for smaller output and the
+    * browser can handle the output just fine.
+    */
+    void setDropNullPlaceholders(bool v);
+    /** \brief Do not add \n at end of document.
+     * Normally, we add an extra newline, just because.
+     */
+    void setOmitEndingLineFeed(bool v);
+    /** \brief Add a space after ':'.
+     * If indentation is non-empty, we surround colon with whitespace,
+     * e.g. " : "
+     * This will add back the trailing space when there is no indentation.
+     * This seems dubious when the entire document is on a single line,
+     * but we leave this here to repduce the behavior of the old `FastWriter`.
+     */
+    void setEnableYAMLCompatibility(bool v);
 
     /// Do not take ownership of sout, but maintain a reference.
     StreamWriter* newStreamWriter(std::ostream* sout) const;