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 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
```

build/_k-xml-docs-test.shade

@@ -0,0 +1,48 @@
+
+use namespace="System"
+use namespace="System.Collections.Generic"
+use namespace="System.IO"
+use import="Files"
+
+default BASE_DIR='${Directory.GetCurrentDirectory()}'
+
+@{
+    var srcDir = Path.Combine(BASE_DIR, "src");
+    foreach (var xmlFilePath in Files.Include(Path.Combine(srcDir, "**/*.xml")))
+    {
+        var errors = 0;
+        var xmlLines = File.ReadAllLines(xmlFilePath);
+        for (var linesIndex = 0; linesIndex < xmlLines.Length; ++linesIndex)
+        {
+            var xmlLine = xmlLines[linesIndex].Trim();
+            if (xmlLine.StartsWith("<!--"))
+            {
+                // Compiler only emits comments for syntax errors.
+                if (errors == 0)
+                {
+                    Log.Warn(string.Format("Invalid documentation syntax in {0}:", xmlFilePath));
+                }
+
+                ++errors;
+                Log.Warn(string.Format("  {0}: {1}", linesIndex + 1, xmlLine));
+            }
+            else if (xmlLine.Contains("\"!:"))
+            {
+                // '!' is reference string error token.
+                if (errors == 0)
+                {
+                    Log.Warn(string.Format("Invalid documentation syntax in {0}:", xmlFilePath));
+                }
+
+                ++errors;
+                Log.Warn(string.Format("  {0}: {1}", linesIndex + 1, xmlLine));
+            }
+        }
+
+        if (errors != 0)
+        {
+            Environment.Exit(errors);
+            return;
+        }
+    }
+}