Automatic demangling of symbols in Emacs buffers

[Per Nordlöw]

The Emacs mode `demangle-mode` [1] can now be used, typically in 
`compilation-minor-mode`, to demangle symbols in Emacs buffers, 
either via `c++filt` or via `ddemangle`.

The demangling executable also just became configurable via [2] 
by customizing the variable `demangle-languages`.

[1] https://github.com/liblit/demangle-mode
[2] 
https://github.com/liblit/demangle-mode/pull/8#issuecomment-699525497