ShingleFilter should have an option to skip filler tokens (e.g. stop words)

ShingleFilterFactory should have an option to ignore filler tokens in the total shingle size.
For instance (adapted from https://stackoverflow.com/questions/33193144/solr-stemming-stop-words-and-shingles-not-giving-expected-outputs), consider the text "A brown fox quickly jumps over the lazy dog". When we remove stopwords and execute the ShingleFilter (shingle size = 3), it gives us the following result:

1. _ brown fox
2. brown fox quickly
3. fox quickly jump
4. quickly jump _
5. jump _ _
6. _ _ lazy
7. _ lazy dog

We can clearly see that the filler token "_" occupies one token in the shingle.
I suppose the returned shingles should be:
1. brown fox quickly
2. fox quickly jump
3. quickly jump lazy
4. jump lazy dog

To maintain backward compatibility, i suggest the creation of an option called "skipFillerTokens" to implement this behavior (note that this is different than using fillerTokens="", since the empty string occupies one token in the shingle)

I've attached a patch for the ShingleFilter class (getNextToken() method), ShingleFilterFactory and ShingleFilterTest clases.