CommonGramsFilter improvements [LUCENE-2841]

Currently CommonGramsFilter expects users to remove the common words around which output token ngrams are formed, by appending a StopFilter to the analysis pipeline. This is inefficient in two ways: captureState() is called on (trailing) stopwords, and then the whole stream has to be re-examined by the following StopFilter.

The current ctor should be deprecated, and another ctor added with a boolean option controlling whether the common words should be output as unigrams.

If common words **are** configured to be output as unigrams, captureState() will still need to be called, as it is now.

If the common words are **not** configured to be output as unigrams, rather than calling captureState() for the trailing token in each output token ngram, the term text, position and offset can be maintained in the same way as they are now for the leading token: using a System.arrayCopy()'d term buffer and a few ints for positionIncrement and offsetd. The user then no longer would need to append a StopFilter to the analysis chain.

An example illustrating both possibilities should also be added.

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Migrated from [LUCENE-2841](https://issues.apache.org/jira/browse/LUCENE-2841) by Steven Rowe (@sarowe), 2 votes, updated May 09 2016
Attachments: [commit-6402a55.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-2841/commit-6402a55.patch)

Start with CommonGramsFilter and its current constructor, then compare the analysis behavior with StopFilter as described. Add the constructor option and update token handling for both unigram settings; document both possibilities with an example and verify that the extra StopFilter is unnecessary when unigrams are disabled.

Written by the indexing model from the issue text.