Add tidy-html5 as submodule
Fix options to work with current version of tidy-html5

Unfortunately tidy-html5 defines the options as enumeration
without specifying the corresponding integer values. When new
options get inserted in between older clients break.
Cleanup

.DS_Store is already specified in the global .gitignore file,
so we don't need this extra one.
Add build script

Running msbuild/xbuild now allows to build both tidy-html5 and
TidyManaged. This should only be necessary when tidy-html5 changed.