Implement share_test command.

Make the tool test json blob produced by default. Add new command share_test - to publish this blob to gist (requires setting github credentials in ~/.planemo.yml) and produce a sharable URL to rich HTML report based on these results.

```
    % planemo test
    ...
    All 1 test(s) executed passed.
    simple_constructs[0]: passed
    % planemo share_test
    File published to Github Gist.
    Raw URL: https://gist.githubusercontent.com/jmchilton/457933350b28a3d1e752/raw/3888694aa109e0928a93ddd414b9acadaf578dc5/index
    Share results with URL: http://galaxyproject.github.io/planemo/tool_test_viewer.html?test_data_url=https://gist.githubusercontent.com/jmchilton/457933350b28a3d1e752/raw/3888694aa109e0928a93ddd414b9acadaf578dc5/index
    Embed results with markdown: [View Tool Test Results](http://galaxyproject.github.io/planemo/tool_test_viewer.html?test_data_url=https://gist.githubusercontent.com/jmchilton/457933350b28a3d1e752/raw/3888694aa109e0928a93ddd414b9acadaf578dc5/index)
```