feat: get previous workday

Added the ability to get the previous workday by passing in a negative
number of days along with the date to start from.

This update also includes some overhauling to allow for the library to
be more resilient to changes in the upstream API.

.github/workflows/test.yml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php-version: ['5.3', '5.4', '5.5', '5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0']
+        php-version: ['5.3', '5.4', '5.5', '5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1']
     steps:
     - name: Checkout
       uses: actions/checkout@v2
@@ -23,7 +23,7 @@ jobs:
     - name: Run Tests
       run: vendor/bin/phpunit --coverage-clover ./coverage.xml
     - name: Upload Coverage
-      if: ${{ matrix.php-version == '8.0' }}
+      if: ${{ matrix.php-version == '8.1' }}
       uses: codecov/codecov-action@v1
       with:
         file: ./coverage.xml