feat: add format input and a single-line problem matcher (#40)

Co-authored-by: Joakim Sørensen <ludeeus@gmail.com>

.github/problem-matcher-gcc.json

@@ -0,0 +1,18 @@
+{
+  "problemMatcher": [
+    {
+      "owner": "shellcheck-gcc",
+      "pattern": [
+        {
+          "regexp": "^(.+):(\\d+):(\\d+):\\s(note|warning|error):\\s(.*)\\s\\[(SC\\d+)\\]$",
+          "file": 1,
+          "line": 2,
+          "column": 3,
+          "severity": 4,
+          "message": 5,
+          "code": 6
+        }
+      ]
+    }
+  ]
+}

.github/problem-matcher-tty.json

@@ -1,7 +1,7 @@
 {
     "problemMatcher": [
       {
-        "owner": "shellcheck",
+        "owner": "shellcheck-tty",
         "pattern": [
           {
               "regexp": "^In\\s(.+)\\sline\\s(\\d+):$",

README.md

@@ -131,3 +131,19 @@ by setting `disable_matcher` to `true`.
      with:
        disable_matcher: true
 ```
+
+## Change output format
+
+Shellcheck can print output in these formats: `checkstyle`, `diff`, `gcc`, `json`, `json1`, `quiet`, `tty`. See some examples [here](https://github.com/koalaman/shellcheck/wiki/Integration#pick-the-output-format-that-makes-your-life-easier).
+Only `tty` and `gcc` produce file annotations via problem matcher, default is `gcc`.
+
+- `tty` has multi-line log messages, but all annotations are reported as errors
+- `gcc` has single-line log messages, so it's easier to parse with a problem matcher (including correct severity annotation)
+
+```yaml
+   ...
+   - name: Run ShellCheck
+     uses: ludeeus/action-shellcheck@master
+     with:
+       format: tty
+```

action.yaml

@@ -26,6 +26,10 @@ inputs:
     description: "Set to true to skip using problem-matcher"
     required: false
     default: "false"
+  format:
+    description: "Output format (checkstyle, diff, gcc, json, json1, quiet, tty)"
+    required: false
+    default: "gcc"
 outputs:
   files:
     description: A list of files with issues
@@ -42,8 +46,9 @@ runs:
     - name: Enable problem-matcher
       shell: bash
       run: |
-        if [[ ${{ inputs.disable_matcher }} != "true" ]]; then
-          echo "::add-matcher::${{ github.action_path }}/.github/problem-matcher.json"
+        problem_matcher_file="${{ github.action_path }}/.github/problem-matcher-${{ inputs.format }}.json"
+        if [[ ${{ inputs.disable_matcher }} != "true" && -f "$problem_matcher_file"  ]]; then
+          echo "::add-matcher::$problem_matcher_file"
         fi
 
     - name: Download shellcheck
@@ -78,6 +83,7 @@ runs:
         if [[ -n "${{ inputs.severity }}" ]]; then
           options+=("-S ${{ inputs.severity }}")
         fi
+        options+=("--format=${{ inputs.format }}")
         echo "::set-output name=options::${options[@]}"
 
     - name: Gather excluded paths