Add an option to filter by issue status

[kinow]

Hi, just found out about govcode.org after watching a Mozilla Science Lab call, kudos!

While browsing for issues, I filtered for Java, and did not select an organisation.

Under "More Issues", the first issue (which is repeated three times?) points to a GitHub issue that has been closed already.

I never wrote Go code, but looking at the existing source code, I think this for loop https://github.com/dlapiduz/govcode.org/blob/8e381f0e9c9aa4c7122b6fc383e3995b9ad9c64b/govcode/import.go#L384 could filter out issues that are not open.