feat(plugins/calendar): add new plugin (#1013) [skip ci]

source/plugins/calendar/metadata.yml

@@ -0,0 +1,22 @@
+name: "📆 Calendar"
+category: github
+description: This plugin displays your commit calendar across several years
+examples:
+  +current year: https://github.com/lowlighter/metrics/blob/examples/metrics.plugin.calendar.svg
+  full history: https://github.com/lowlighter/metrics/blob/examples/metrics.plugin.calendar.full.svg
+supports:
+  - user
+scopes:
+  - public_access
+inputs:
+
+  plugin_calendar:
+    description: Enable calendar plugin
+    type: boolean
+    default: no
+
+  plugin_calendar_limit:
+    description: Years to display
+    type: number
+    default: 1
+    zero: disable