Add ASCII armor support

[csssuf]

ASCII armor is widely used, so we should support it.

Ideally we'd support it transparently, so we should add support for it directly to the parsers in src/packet.rs. We also want to support writing it, so a new function to_armor should be added to Packet.

Reading through RFC4880 section 6 would be a good start here. For an initial implementation throwing away the armor headers would be acceptable, but eventually we'll want to support those too, which will require some refactoring of Packet.