Add CLI command to add features

[garrettjstevens]

When trying to test #747, I realized there wasn't a CLI command to add a top-level feature. I needed one to properly test that PR, and it seems like a general-purpose command, so I added it.

You can add a simple feature with the --min, --max, --type, etc. flags, or a more complex feature from JSON, which can be in a file or read from stdin.

apollo feature add <<EOF
{
  "assembly": "vv1",
  "refSeq": "ctgA",
  "min": 301,
  "max": 310,
  "type": "match",
  "attributes": {"gff_id": ["match1"]},
  "children": [
    {
      "min": 301,
      "max": 305,
      "type": "match_part",
      "attributes": {"gff_id": ["matchPart1"]}
    }
  ]
}
EOF