Add matrix to GitHub workflow (#105)

- Add matrix with OS and Go versions to GitHub workflow.
- Add codecov flag.

Author: pellared

.github/workflows/build.yml

@@ -10,7 +10,14 @@ on:
 
 jobs:
   ci-build:
-    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        go-version: [1.15]
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
+    defaults:
+      run:
+        shell: bash
     steps:
       - uses: actions/checkout@v2
         with:
@@ -21,7 +28,7 @@ jobs:
           key: ${{ runner.os }}-go-${{ hashFiles('**/go.sum') }}
       - uses: actions/setup-go@v2
         with:
-          go-version: 1.15
+          go-version: ${{ matrix.go-version }}
       - name: Build
         run: make ci
       - name: Upload coverage
@@ -38,3 +45,4 @@ jobs:
         uses: codecov/codecov-action@v1
         with:
           file: ./coverage.out
+          flags: ${{ runner.os }}