Use some features only in certain modes

Use fontify meta, multiple function signatures and post completion templatifying only in some modes.

The inforamtion in the property `detailed_info` is formatted differently in python than in c++. I c++ it contains a list with the function signature or multiple signatures. In python it includes also the docstring and therefore it does not make sense to fontify this string in the minibuffer.

The other reason for this patch is https://github.com/abingham/emacs-ycmd/issues/115 and the fact that post completion templatifying does not work for python properly because the format of `detailed_info` is not suited for extracting the function parameters. Also the ycmd response does not contain `menu_text` for python, which is used as fallback to extract function parameters.

To fix https://github.com/abingham/emacs-ycmd/issues/115 extracting function parameters should be implemented properly for python and therefore I suggest to disable the feature for python entirely.
Unless we expect the user to change this, shouldn't it be `defconst`? And perhaps named `company-ycmd--extended-features-mode`?