Add shellcheck to pre-commit

Closes #22

.pre-commit-config.yaml

@@ -23,4 +23,9 @@ repos:
         description: disallow checking in SHOW_MEMORY=1
         entry: '^#define SHOW_MEMORY 1$'
         language: pygrep
-        types: [c++]
+        types: [c++]
+  - repo: https://github.com/koalaman/shellcheck-precommit
+    rev: 2491238703a5d3415bb2b7ff11388bf775372f29  # frozen: v0.10.0
+    hooks:
+    -   id: shellcheck
+#       args: ["--severity=warning"]  # Optionally only show errors and warnings