Several new plugins/modules implemented

I've made some additions to Maddy that were essential for my own deployment; I'm not sure if any of these would be useful to anyone else, or appropriate for the Maddy core, but I thought I'd offer them up for review just in case. I wouldn't consider them heavily battle-tested, but they've been in production for about 6 months on my own low-volume mail server.

Complete change list is [here](https://github.com/foxcpp/maddy/compare/master...sblinch:maddy:master).

Some highlights:

- [pattern check plugin](https://github.com/foxcpp/maddy/commit/28f5b6f544d78eeb7dc01dff5adb02b16ebc7f7b), to whitelist/blacklist senders, recipients, hosts, and headers by pattern, regexp, or CIDR
- [spamassassin check plugin](https://github.com/foxcpp/maddy/commit/67651264ce43ffa32f72e8c6df32381d0ef1d80d), adding SA support
- [geobl check plugin](https://github.com/foxcpp/maddy/commit/be705f687a10e87ebd4a956db91b292e253bedb3), to ban countries by connecting IP
- [domainbl check plugin](https://github.com/foxcpp/maddy/commit/033cbe00b9a9bb01b83761829a48580ee82ea95d), to support SURBL/URIBL/etc
- [safelist support](https://github.com/foxcpp/maddy/commit/365a658fe538a3d5790782469d95e841f0bb9cdd) to add "safelist" in addition to "reject", "ignore", etc., which overrides all other check plugins and ensures a message is delivered
- [crypto storage module](https://github.com/foxcpp/maddy/commit/13f0b5736a96f61a0f7d748079b5f9045d8e40b7), a wrapper that transparently adds encryption support to any other storage blob module
- [delivery attempt logging](https://github.com/foxcpp/maddy/commit/b67e6ab6630430bb9365b127e31347385915c60d), to more easily grep for specific delivery incidents in production logs
- dnsbl improvements to [log which dnsbl providers returned hits, and why](https://github.com/foxcpp/maddy/commit/b9b9338be75863ed8e8286f790c476e40e46e653) and [be more resiliant to dnsbl outages](https://github.com/foxcpp/maddy/commit/cd9bd1ba7b3a5fc2d4ce6b7cc1d1a74fe31f5c13)
- [table storage blob module](https://github.com/foxcpp/maddy/commit/c7c8102cbc6fc2e414ebc84136b31be1c079faca), to use any table module (such as sqlite) as blob storage
- [require_matching_rdns_lax check plugin](https://github.com/foxcpp/maddy/commit/63ae3ff4185421384736ff39ecd805d6afcf9cce), handles a commonly-encountered corner case for require_matching_rdns (not sure if this is even relevant any more; I see there were some changes involving this code since I originally implemented it)

For documentation, for now, I've included a doc.go for each plugin/module that's formatted similarly to docs/reference/checks/*.md.