Use file extension to select config file parser

This patch makes Dotbot provide better error messages when parsing JSON
files.

dotbot/config.py

@@ -1,5 +1,6 @@
 import yaml
 import json
+import os.path
 from .util import string
 
 class ConfigReader(object):
@@ -8,17 +9,13 @@ class ConfigReader(object):
 
     def _read(self, config_file_path):
         try:
+            _, ext = os.path.splitext(config_file_path)
             with open(config_file_path) as fin:
-                try:
-                    data = yaml.safe_load(fin)
-                except Exception as e:
-                    # try falling back to JSON, but return original exception
-                    # if that fails too
-                    try:
-                        fin.seek(0)
+                print ext
+                if ext == '.json':
                     data = json.load(fin)
-                    except Exception:
-                        raise e
+                else:
+                    data = yaml.safe_load(fin)
             return data
         except Exception as e:
             msg = string.indent_lines(str(e))