add HTML Validation

[markusfalk]

https://github.com/jzaefferer/grunt-html

[markusfalk]

praveenvijayan/grunt-html-validation#76

[markusfalk]

would prefer solution without local java but couldn't get it to work without network error

[SpikeShape]

Wanna try jzaefferer/grunt-html?

According to this answer it uses a 'local copy of the java-based The Nu HTML Checker'.

Its still not a great solution since it depends on java but its better than calling a remote service imho.

[markusfalk]

@SpikeShape I tried this already. see comment #148 (comment)

Don't know if it is a good idea to add another dependency to the project. Not a fan of adding another online service either.