feat: update changelog (#131)

closes #130 

- CHANGELOG.md file for people who can view changelog from github
---
`iris changelog` command:
`iris changelog [version]` (e.g. iris changelog v0.5.1)
`iris changelog -n 3` (see changelog of 3 latest releaseas)

=> `iris changelog` looks just like CHANGELOG.md, grouped headers and
all. It only stops looking that way if we personally overwrite that
release's description with something else
=> With 1-hour local cache (invalidated on updater-channel change),
`--refresh` flag to bypass, graceful fallback to stale cache on GitHub
rate-limiting

---
- New auto update config
```toml
# 0 = off (default, notify only), 1 = auto-install, 2 = always confirm first
auto-update = 1
```

- New just debug-changelog and just debug-autoupdate recipes (fully
offline, mocked HTTP servers, no real network/config touched)

.goreleaser.yaml

@@ -14,6 +14,35 @@ git:
     # already has them and fails with 422 already_exists
     prerelease_suffix: "-nightly"
 
+changelog:
+    use: git
+    sort: asc
+    abbrev: 6
+    format: "{{ .SHA }}  {{ .Message }}"
+    filters:
+        # these would otherwise fall under the "Bug fixes" group below since
+        # they start with `fix`, but they're infra-only and not user-facing
+        exclude:
+            - "(?i)^fix\\(ci\\)"
+            - "(?i)^fix\\(release\\)"
+    groups:
+        - title: "Features"
+          regexp: '(?i)^.*?feat(\(.+\))?!?:.+$'
+          order: 0
+        - title: "Bug fixes"
+          regexp: '(?i)^.*?fix(\(.+\))?!?:.+$'
+          order: 1
+        - title: "Performance"
+          regexp: '(?i)^.*?perf(\(.+\))?!?:.+$'
+          order: 2
+        - title: "Refactors"
+          regexp: '(?i)^.*?refactor(\(.+\))?!?:.+$'
+          order: 3
+        - title: "Documentation"
+          regexp: '(?i)^.*?docs?(\(.+\))?!?:.+$'
+          order: 4
+        # unmatched types (chore, build, ci, test, ...) are silently dropped
+
 builds:
     - env:
           - CGO_ENABLED=0