Optionally log warnings to Checkstyle XML

[totten]

"Checkstyle XML" is a quasi-standard format used by tools like phpcs, jshint, or Jenkins to share a list of code-formatting issues. Here's a representative example of the format:

<?xml version="1.0" encoding="UTF-8"?>
<checkstyle version="3.7.1">
<file name="/private/tmp/z.php">
 <error line="4" column="3" severity="error" message="The extends keyword must be on the same line as the class name" source="Drupal.Classes.ClassDeclaration.ExtendsLine"/>
</file>
</checkstyle>

Currently, civistrings does some validation and prints warnings - but it doesn't give structured information.

Ideally, there would be an option to send these warnings to a file (civistrings --checkstyle=output.xml *.php).