Add ClusterFuzzLite integration

.clusterfuzzlite/Dockerfile

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+FROM gcr.io/oss-fuzz-base/base-builder
+RUN apt-get update && apt-get install -y make autoconf automake libtool
+
+COPY . $SRC/maddy
+COPY .clusterfuzzlite/build.sh $SRC/build.sh
+WORKDIR $SRC/maddy

.clusterfuzzlite/README.md

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+# ClusterFuzzLite set up
+
+This folder contains a fuzzing set for [ClusterFuzzLite](https://google.github.io/clusterfuzzlite).

.clusterfuzzlite/build.sh

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+#!/bin/bash -eu
+
+# Copy all fuzzer executables to $OUT/
+$CXX $CFLAGS $LIB_FUZZING_ENGINE \
+  $SRC/maddy/.clusterfuzzlite/parser_fuzzer.cpp \
+  -o $OUT/parser_fuzzer \
+  -I$SRC/maddy/include

.clusterfuzzlite/parser_fuzzer.cpp

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+#include "maddy/parser.h"
+#include <string>
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  std::string payload(reinterpret_cast<const char *>(data), size);
+  auto parser = std::make_shared<maddy::Parser>();
+  std::stringstream markdown(payload);
+  parser->Parse(markdown);
+
+  return 0;
+}

.clusterfuzzlite/project.yaml

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+language: c++

.github/workflows/cflite_pr.yml

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+name: ClusterFuzzLite PR fuzzing
+on:
+  workflow_dispatch:
+  pull_request:
+    branches: [ master ]
+permissions: read-all
+jobs:
+  PR:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        sanitizer: [address]
+    steps:
+    - name: Build Fuzzers (${{ matrix.sanitizer }})
+      id: build
+      uses: google/clusterfuzzlite/actions/build_fuzzers@v1
+      with:
+        sanitizer: ${{ matrix.sanitizer }}
+        language: c++
+        bad-build-check: false
+    - name: Run Fuzzers (${{ matrix.sanitizer }})
+      id: run
+      uses: google/clusterfuzzlite/actions/run_fuzzers@v1
+      with:
+        github-token: ${{ secrets.GITHUB_TOKEN }}
+        fuzz-seconds: 180
+        mode: 'code-change'
+        report-unreproducible-crashes: false
+        sanitizer: ${{ matrix.sanitizer }}