Fix vim file_directory segment

Closes #117.
Closes #119.

powerline/segments/vim.py

@@ -81,6 +81,8 @@ def readonly_indicator(text=u''):
 def file_directory():
 	'''Return file directory (head component of the file path).'''
 	file_directory = vim_funcs['expand']('%:~:.:h')
+	if file_directory is None:
+		return None
 	if file_directory.startswith('/home/'):
 		file_directory = '~' + file_directory[6:]
 	return file_directory.decode('utf-8') + os.sep if file_directory else None