Move options to test runner,  plugins has priority and app parser has the test runner parser

In this pull request, many command line options that was once inside the main application, like the json and xunit outputs, are now moved into the `run` subcommand (the test runner). There are important internal consequences after this user change:

1) All plugins now contains an attribute `priority` which is relevant in the time the plugins are loaded, the lowest the value, the first to be loaded. By now, only the test runner has priority 0, all other plugins got priority 3.

2) The test runner command line parser is now contained in the parser instance of the application (`parser.runner`), so the other plugins can easily extend the runner. Which is the case of xunit, json, vm, etc. plugins. Example: `self.parser.runner.add_argument(...)`.