[feat] Remove support for old 'output' setting

[alde]

Breaking change

Remove the output setting, and change the default of outputDirectory
from null to process.cwd().

This will cause the defaults for outputDirectory and outputName to
be joined together to the same as the old output value, with the added
benefit of them being overrideable separately.

Fixes #100

[palmerj3]

Thank you!

[literalpie]

An example that people may find useful:

We have a project that did have a configuration of: "output": "./test-results/xml-reports/unit-results.xml",
With this change, the configuration is now:

{
    "outputDirectory": "./test-results/xml-reports",
    "outputName": "unit-results.xml",
    ...
}