Feature: Add File Pattern Input

[stefanzweifel]

Fixes #12

Adds a new file_pattern input, which allows user to customize, which files should be added to the git commit.

Example usage:

    - uses: stefanzweifel/[email protected]
      with:
        commit_message: Commit Message
        branch: ${{ github.head_ref }}
        file_pattern: src/\*.js
      env:
        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}