Add dump_format=json option (#1726)

* Add format to the params accepted by DumpModel

Currently, only the test format is supported when trying to dump
a model. The plan is to add more such formats like JSON which are
easy to read and/or parse by machines. And to make the interface
for this even more generic to allow other formats to be added.

Hence, we make some modifications to make these function generic
and accept a new parameter "format" which signifies the format of
the dump to be created.

* Fix typos and errors in docs

* plugin: Mention all the register macros available

Document the register macros currently available to the plugin
writers so they know what exactly can be extended using hooks.

* sparce_page_source: Use same arg name in .h and .cc

* gbm: Add JSON dump

The dump_format argument can be used to specify what type
of dump file should be created. Add functionality to dump
gblinear and gbtree into a JSON file.

The JSON file has an array, each item is a JSON object for the tree.
For gblinear:
 - The item is the bias and weights vectors
For gbtree:
 - The item is the root node. The root node has a attribute "children"
   which holds the children nodes. This happens recursively.

* core.py: Add arg dump_format for get_dump()

include/xgboost/c_api.h

@@ -446,6 +446,23 @@ XGB_DLL int XGBoosterDumpModel(BoosterHandle handle,
                                bst_ulong *out_len,
                                const char ***out_dump_array);
 
+/*!
+ * \brief dump model, return array of strings representing model dump
+ * \param handle handle
+ * \param fmap  name to fmap can be empty string
+ * \param with_stats whether to dump with statistics
+ * \param format the format to dump the model in
+ * \param out_len length of output array
+ * \param out_dump_array pointer to hold representing dump of each model
+ * \return 0 when success, -1 when failure happens
+ */
+XGB_DLL int XGBoosterDumpModelEx(BoosterHandle handle,
+                                 const char *fmap,
+                                 int with_stats,
+                                 const char *format,
+                                 bst_ulong *out_len,
+                                 const char ***out_dump_array);
+
 /*!
  * \brief dump model, return array of strings representing model dump
  * \param handle handle
@@ -465,6 +482,27 @@ XGB_DLL int XGBoosterDumpModelWithFeatures(BoosterHandle handle,
                                            bst_ulong *out_len,
                                            const char ***out_models);
 
+/*!
+ * \brief dump model, return array of strings representing model dump
+ * \param handle handle
+ * \param fnum number of features
+ * \param fname names of features
+ * \param ftype types of features
+ * \param with_stats whether to dump with statistics
+ * \param format the format to dump the model in
+ * \param out_len length of output array
+ * \param out_models pointer to hold representing dump of each model
+ * \return 0 when success, -1 when failure happens
+ */
+XGB_DLL int XGBoosterDumpModelExWithFeatures(BoosterHandle handle,
+                                             int fnum,
+                                             const char **fname,
+                                             const char **ftype,
+                                             int with_stats,
+                                             const char *format,
+                                             bst_ulong *out_len,
+                                             const char ***out_models);
+
 /*!
  * \brief Get string attribute from Booster.
  * \param handle handle