Support the specification of a directory containing the gatorgrade.yml file

[gkapfham]

Hello @mariakimheinert and @Michionlion and @burgess01!

Thanks for your work on this GitHub Action, I really appreciate it.

I noticed that in the documentation and in the actual implementation of this GitHub Action it seems as though you cannot specify the directory that contains the gatorgrade.yml file. For instance, the following example does not allow the specification of a directory that would contain the gatorgrade.yml file. Would this be possible to add? Thanks for any suggestions that you have!

name: Grader
on: [push]
jobs:
    grade:
        run-on: ubuntu-latest
        steps:
            - name: Checkout repository
                uses: actions/checkout@v2
            - name: Run Gatorgrade
                uses: GatorEducator/gatorgrade-action@v1