Add recipe for propfont-mixed.

It is a minor mode for using proportional-width fonts alongside
fixed-width space-based indentation.
Merge pull request #2316 from ikirill/master

Add recipe for propfont-mixed.
Add irony-eldoc.

It is a minor mode that enables eldoc-mode support (showing docstrings
in minibuffer) for C/C++/ObjC, based on irony-mode, which is based on
libclang. Based on irony-mode and libclang, it is quite precise (except
for ambiguous function calls), and avoids recompilation of files.