Support using default config on no path match

[mcncl]

Changes

Issue #23 raises a good point of having a default config, which may be a nice addition to the plugin and allow folks to not have to cover all bases to run a config (command or trigger)

• Supports a default config in the plugin

This can be used as either:

    ...
        - default:
            config:
              command: buildkite-agent pipeline upload other_tests.yml
    ...

or:

    ...
        - default:
            trigger: cool-pipeline
    ...

• Added tests to cover the cases where default may be used

[tomowatt]

[mcncl]

I’ll update the README in the morning @tomowatt, so will leave this unmerged for now