Support multiple files in checkstyle formatter

[fsahmad]

PR checklist

• Addresses an existing issue: #0000
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

What changes did you make?

Found an issue when integrating tslint with jenkins (using checkstyle formatter), the formatter did not generate separate file nodes in the outputted XML, instead simply reporting all errors under the first file.

To fix this I:

• Modified checkstyleFormatter.ts to sort failures by filename, and generate file nodes for each unique filename
• Modified test case to test failures in multiple files

Is there anything you'd like reviewers to focus on?

The checkstyleFormatter test was reusing another formatter's test input file, I've followed this pattern to add a second file in the test. It might be better to create two test input files specifically for the checkstyleFormatter...

[palantirtech]

Thanks for your interest in palantir/tslint, @fsahmad! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[nchen63]

@fsahmad thanks!