Group project warnings

[simendsjo]

This is regarding dub 0.9.12

Great that you print warnings for projects, but there's a couple of issues.

1. The project name is quite hidden in the text
2. One warning block is printed per warning

So rather than this:

Warning
=======

The following compiler flags have been specified in pegged's
package description file. They are handled by DUB and direct use in packages is
discouraged.
Alternatively, you can set the DFLAGS environment variable to pass custom flags
to the compiler, or use one of the suggestions below:

-wi: Managed by DUB, do not specify in package.json
-property: Managed by DUB, do not specify in package.json


Warning
=======

The following compiler flags have been specified in pegged's
package description file. They are handled by DUB and direct use in packages is
discouraged.
Alternatively, you can set the DFLAGS environment variable to pass custom flags
to the compiler, or use one of the suggestions below:

-ggdb: Call dub with --build=debug

( and it goes on and on and on .. )

I wold like something like this:

Warning [project = pegged]
=======
The following compiler flags have been specified in the projects
package description file. They are handled by DUB and direct use in packages is
discouraged.
Alternatively, you can set the DFLAGS environment variable to pass custom flags
to the compiler, or use one of the suggestions below:

-wi: Managed by DUB, do not specify in package.json
-property: Managed by DUB, do not specify in package.json
-ggdb: Call dub with --build=debug

[s-ludwig]

The output would now be like this:

## Warning for package pegged ##

The following compiler flags have been specified in the package description
file. They are handled by DUB and direct use in packages is discouraged.
Alternatively, you can set the DFLAGS environment variable to pass custom flags
to the compiler, or use one of the suggestions below:

-wi: Managed by DUB, do not specify in package.json
-property: Managed by DUB, do not specify in package.json
-ggdb: Call dub with --build=debug

It warned for every "dflags-" field again and multiple times for each package. It is now only checked once in a central place and all "dflags-" fields are first merged.