set default yapf parameters to use yafp style

Add lines to `kernel_exec_on_cell` cfg that mimic 
[yapf command line behavior](https://github.com/google/yapf/blob/f672d7b1073c29054cfcc9c32f344c948e8cf1bf/yapf/__init__.py#L142) 
for loading global and local styles.

Author: artificialsoph

src/jupyter_contrib_nbextensions/nbextensions/code_prettify/code_prettify.js

@@ -26,9 +26,12 @@ define(['./kernel_exec_on_cell'], function(kernel_exec_on_cell) {
             "library": ["import json",
             "def yapf_reformat(cell_text):", 
             "    import yapf.yapflib.yapf_api",
+            "    from yapf import file_resources",
+            "    import os",
             "    import re",
+            "    style_config = file_resources.GetDefaultStyleForDir(os.getcwd())",
             "    cell_text = re.sub('^%', '#%#', cell_text, flags=re.M)",
-            "    reformated_text = yapf.yapflib.yapf_api.FormatCode(cell_text)[0]",
+            "    reformated_text = yapf.yapflib.yapf_api.FormatCode(cell_text, style_config=style_config)[0]",
             "    return re.sub('^#%#', '%', reformated_text, flags=re.M)"].join("\n"),
             "prefix": "print(json.dumps(yapf_reformat(u",
             "postfix": ")))"