feat(panettone): render a subset of markdown in issue subjects

This is achieved by implementing a simple markdown renderer in CL which
has the following limitations:

* Only supports inline `code`, *emphasize 1*, _emphasize 2_ and
  ~~strikethrough~~.
* Does not support nested markup.

This allows for a relatively simple renderer which doesn't need to parse
markdown into a in-memory data structure first. The rendered result is
directly written to a stream to integrate well with cl-who which is also
reused for rendering tags and xml-escaping strings.

Fixes #90.

Change-Id: Ice88ed770b1fab6365f3b93e8663e25077befa0b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2389
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>

web/panettone/test/package.lisp

@@ -1,2 +1,3 @@
 (defpackage :panettone.tests
-  (:use :cl :klatre :fiveam))
+  (:use :cl :klatre :fiveam
+        :panettone.inline-markdown))