Support passing `--srcdir` to goimports

When using goimports via our gofmt formatter as [suggested in the docs](https://github.com/google/vim-codefmt/blob/1025977/doc/codefmt.txt#L42), the goimports path-based inference doesn't work because we don't tell goimports about the path. It does support a `--srcdir` argument we could pass.

We'd have to make it conditional and detect `basename(gofmt_executable) == 'goimports'` or something like that because gofmt doesn't support any `--srcdir` arg.