Allow changelog to work in a given subdirectory

ahoy.changelog.yml

 ahoyapi: v2
 commands:
   extract:
-    cmd: git log $(git describe --tags `git rev-list --tags --max-count=1`)..HEAD --oneline | cut -d' ' -f 2-999
-    usage: Extract changelog from Git for the current project since the last release.
+    cmd: |
+      if [ "x$1" <> "x" ]; then
+        cd $1
+      fi
+      git log $(git describe --tags `git rev-list --tags --max-count=1`)..HEAD --oneline | cut -d' ' -f 2-999
+    usage: Extract changelog from Git for the current project (or the given path as 1st argument) since the last release.