[pkg-go] dh-golang : DH_GOLANG_INSTALL_EXTRA_EXTENSIONS option

[Alexandre Viau]

> I think the go tools do not cover files that are referenced by test code, e.g.:

Also, this does not to include files that are used in go:generate
calls. For example, .proto files.

Note that my proposed patch would also allow to specify a file name,
for example ``test.sh`` and it would be installed.

We can extend the default list of installed extensions, but I'm not
sure that I would like a black list. Simply because it would be more
complicated to maintain and I prefer to list what I want instead of
what I don't. However, that's just a matter of preference.