Include body parameter for release

Fixes #4

This adds the ability to include the body parameter when creating the
release.

    - name: Checkout code
      uses: actions/checkout@master
    - name: Create Release
      id: create_release
      uses: actions/create-release@master
      with:
        tag_name: ${{ github.ref }}
        release_name: Release ${{ github.ref }}
        body: Release body

This also supports a multiline body:

    - name: Create Release
      id: create_release
      uses: actions/create-release@master
      with:
        tag_name: ${{ github.ref }}
        release_name: Release ${{ github.ref }}
        body: |
          This is a multiline body
          with more than one
          line

Or if you want the contents of a file:

    - name: Read CHANGELOG
      id: changelog
      run: |
        echo "::set-output name=body::$(cat CHANGELOG.md)"
    - name: Create Release
      id: create_release
      uses: actions/create-release@master
      with:
        release_name: Release ${{ github.ref }}
        body: ${{ steps.changelog.outputs.body }}

One decision I made in favor of less code was to send an empty body when
there was non present. If it would be preferred to send nothing in the
request for the `body` attribute I could instead compose the parameters
with an optional body attribute:

    releaseParams = {
      owner,
      repo,
      tag_name: tag,
      name: releaseName,
      draft,
      prerelease
    };

    if (body) releaseParams.body = body;

    const createReleaseResponse = await github.repos.createRelease( releaseParams );

src/create-release.js

@@ -15,6 +15,7 @@ async function run() {
     // This removes the 'refs/tags' portion of the string, i.e. from 'refs/tags/v1.10.15' to 'v1.10.15'
     const tag = tagName.replace('refs/tags/', '');
     const releaseName = core.getInput('release_name', { required: true }).replace('refs/tags/', '');
+    const body = core.getInput('body', { required: false });
     const draft = core.getInput('draft', { required: false }) === 'true';
     const prerelease = core.getInput('prerelease', { required: false }) === 'true';
 
@@ -26,6 +27,7 @@ async function run() {
       repo,
       tag_name: tag,
       name: releaseName,
+      body,
       draft,
       prerelease
     });