Introduce checkstyle and JUnit output formats for lint command

[sandermvanvliet]

This PR introduces two new output formats for the lint command:

• checkstyle
• JUnit

These formats can be selected by providing a -o junit argument to speccy lint.

Console output remains the default but has been moved to a separate renderer as well to remove the concern for output rendering form the lint command.

For all output types except console there is an additional argument, -f, to specify the file the output should be written to:

user@host$> speccy lint -o junit -f validation-results.xml spec-to-validate.json

This writes a JUnit result file to validation-results.xml which allows the build system to pick this up and add to the build results.