Add a formatting-option

[hpbuniat]

The most tools which are started via testy can output xml (junit, checkstyle, pmd) instead of plain-text. As e.g. growl & libnotify do have limited space, testy should have the ability to parse this xml and create a output with better readability.

• configure output-formatter per project
• use a fallback (PLAIN) formatter, if the configured output-formatter fails during parsing