Update project mithril to v0.1.21

libgrabber updated project mithril to v0.1.21.

@lhorie

### Metadata information

```json
{
  "metadata": {
    "packageManager": "github",
    "name": "mithril",
    "repo": "lhorie/mithril",
    "files": {
      "include": ["mithril.js", "mithril.min.js", "mithril.min.map"],
      "exclude": []
    },
    "path": "/home/jsdelivr/files/mithril",
    "localVersions": ["0.1.0", "0.1.1", "0.1.2", "0.1.3", "0.1.4", "0.1.5", "0.1.6", "0.1.7", "0.1.8", "0.1.9", "0.1.10", "0.1.11", "0.1.12", "0.1.13", "0.1.14", "0.1.15", "0.1.16", "0.1.17", "0.1.18", "0.1.19", "0.1.20"],
    "remoteVersions": ["v0.1.0", "v0.1.1", "v0.1.2", "v0.1.3", "v0.1.4", "v0.1.5", "v0.1.6", "v0.1.7", "v0.1.8", "v0.1.9", "v0.1.10", "v0.1.11", "v0.1.12", "v0.1.13", "v0.1.14", "v0.1.15", "v0.1.16", "v0.1.17", "v0.1.18", "v0.1.19", "v0.1.20", "v0.1.21"],
    "branchVersions": []
  },
  "version": "v0.1.21",
  "updated": true,
  "updatePath": "/home/jsdelivr/files/mithril/0.1.21",
  "branch": "mithril/v0.1.21"
}
```