Categorization of warnings

[Johan Engelen via Digitalmars-d]

I just submitted a PR [1] that catagorizes warnings, such that 
you can do something like this:
dmd -w -Wno-not-reachable
which would error on any warning except the "statement not 
reachable" warnings (it completely disables that warning).

The motivation for the selective disabling/enabling of warnings 
was a recent discussion in the Learn forum [2].

Please read about it in detail in the first message of the PR [1].

- Johan

[1]  https://github.com/D-Programming-Language/dmd/pull/5592
[2]  
http://forum.dlang.org/thread/baupegcfvumouhgauetk@forum.dlang.org