[ANN] DefaultApplication.jl

Thanks to everyone who helped with testing, DefaultApplication.jl is now registered. It only works on v0.7; but a backport should be trivial, just open an issue if you need it.

The main feature of the package is opening a file with the default application determined by the OS:

import DefaultApplication

should open a file in your default image viewer, etc.

This is implemented by a rather trivial function; the purpose of this package is to have it in one place, updating it as the OS interface changes, so that package authors don’t have to worry about this.

The package also includes a DefaultApplication.test() function, which tests the functionality and prints out debugging information.