more tests for attachments

[springmeyer]

We appear to have only one test in test/rendering that uses attachments and it was added for #32. We should add more.

[springmeyer]

/cc @yohanboniface @gravitystorm @ajashton @skorasaurus - could you all please make some time to add more tests of existing, working styles? Now that it appears we have fixes the known regressions, it is important to ensure we have better test coverage going forward to make future refactoring and optimizations easier.

To add tests of standalone mss files simply do:

1. add a new file to test/rendering-mss/<filename>.mss
2. run npm test
3. the tests should automatically generate an expected result at test/rendering-mss/<filename>.xml
4. git add test/rendering-mss/* && git commit -a -m "add more mss tests"
5. provide a pull request

[springmeyer]

a couple more added in 45fae55 /cc @tmcw

[springmeyer]

ping @yohanboniface @gravitystorm @ajashton @skorasaurus - anyone up for contributing a few mss files that use attachments to help prevent regressions? (see my notes above from contributing tests).

[springmeyer]

thanks @yohanboniface for the tests. Everyone else, please don't forget to offer more tests when you think of it.