Suggestion: __traits(ddoc)

[James Lu]

DDoC is very convenient for generating HTML documentation.

Sometimes, though, we need to print documentation back, for 
example when writing a command line tool and "-h" is passed.

I suggest `__traits(ddoc, symbol)`, which would return the 
concatenated DDoC source of a symbol as an enum string.

If DMD compiler authors provide pointers (where to start, what 
methods to call) on how to do this, I will implement this myself 
and send a pull request.