support staticcheck configuration

[benjaminjkraft]

Staticcheck has a bunch of options. We'd like to be able to use those via golangci-lint, but it doesn't seem to be supported.

It's not clear to me if staticcheck exposes a proper way to do this, but it looks like setting DefaultConfig would be a plenty usable hack.

I think this would subsume #649, #980, perhaps others.

[csilvers]

It looks like staticcheck-in-the-context-of-golangci-lint reads the staticcheck.conf file, so another usable hack is to just set that. It's not great to have two config files but it's better than nothing.