Add base input argument to only annotate diff'ed files

[charlespwd]

In this PR, we add support for the base input argument. It can be a git branch, tag or commit sha.

When this is set as well as the token input argument, only the files containing diffs will receive GitHub check annotations.

This should help curb the 1000 annotation limit for folks dealing with legacy themes.

Example configuration:

  theme-check:
    name: Theme Check
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - name: Theme Check
        uses: shopify/theme-check-action@v1
        with:
          token: ${{ github.token }}
          base: main

• Demo run with failures
• Demo run without failures

Fixes #3