Validator: add initializer

There is a method to add a set of tags to a validator on initialisation.
First this removes a user application from the burden of maintaining the
html spec and also is more performant, as a lot of tags are to be
inserted at once, so there aren't multiple allocation calls.
As the validator needs the tags to be in order, the tags must be sorted
on insertion. Of course it would be easier for the code, if the tags
were already in order, but first there could be easily a mistake and
second sorting the tags by an algorithm allows the tags to be specified
in a logically grouped and those more maintainable order.
For the sorting, insertion sort is used. Of course it has a worse
quadratic time complexity, but in a constructor, I wouldn't introduce
the overhead of memory managment a heap- or mergesort would introduce
and in-place sorting is also out, because the data lies in ro-memory.
Thus I choose an algorithm with constant space complexity. Also the
'long' running time is not so important, as the initilization only runs
at startup once and the tags are not likely to exceed a few hundred so
even a quadratic time isn't that bad.