Add stories about how to extend Canvas with plugins (elastic#214)

* feat: add metallic support

compile formatted markup from markdown

* feat: add story about writing a canvas plugin

use the github-tags function as the example

* feat: add a task to build the site without rebuilding functions

* feat: add part 2 of creating plugins

* chore: update microsite

package.json

@@ -95,6 +95,7 @@
     "metalsmith-filter": "^1.0.2",
     "metalsmith-in-place": "^1.4.4",
     "metalsmith-layouts": "^1.4.1",
+    "metalsmith-metallic": "^2.0.2",
     "metalsmith-permalinks": "^0.5.0",
     "pegjs": "^0.10.0",
     "proxyquire": "1.7.11",