Add shellcheck step to github actions

This runs most of the shellcheck tests. The excludes are:
- SC1091: Avoids errors when shellcheck can't find sourced file
- SC2034: Unused variables, for example colors that aren't used yet
- SC2012: use of ls, from what I can see in our case this is fine (wc -l)

.github/workflows/unit_test.yml

@@ -66,3 +66,14 @@ jobs:
       run: if [[ ${{ matrix.python-version }} == 3.9 ]]; then bash <(curl -s https://codecov.io/bash); fi
       env:
         CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+  shellcheck:
+    name: Shellcheck
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Shell check mycroft tools
+      uses: ludeeus/action-shellcheck@master
+      env:
+        SHELLCHECK_OPTS: -x --exclude SC2034 --exclude SC2012 --exclude SC1091
+      with:
+        scandir: ./bin