Support test for XML documentation errors

- can't be done as a unit test because `k.cmd test` doesn't write assemblies for referenced projects
- instead support extending the "test" target in builds

To enable this in a (clean) repo, add the following to the makefile.shade file:
```
#xml-docs-test target='test'
  k-xml-docs-test
```

Example output with one syntax error and one invalid reference:
```
info: Target xml-docs-test
warn: Invalid documentation syntax in src\Microsoft.AspNet.Mvc.Core\bin\debug\net45\Microsoft.AspNet.Mvc.Core.xml
warn:  3170: <!-- Badly formed XML comment ignored for member "T:Microsoft.AspNet.Mvc.Rendering.HtmlHelper" -->
warn:  3203: If the object is already an <see cref="!:IDictionaries&lt;string, object&gt;"/> instance, then it is
```
Wouldn't this need to be a target after build?