create ArtifactFormat version 7

[gregcaporaso]

Add items to include here.

• conda environment info, if available
• file size of each input artifact
• cpu architecture and flags
• forward compatibility marker of some sort
• support for adding annotation to artifacts (even if we don't have the tools to actually add that yet) - @gregcaporaso and @ebolyen talked about adding an annotations directory with files named something like note-001.md, note-002.md, ..., note-999.md. some reasonable limit would need to be placed on file size for each of these.