Make LowerCaseFilter and StopFilter keyword aware, similar to PorterStemFilter [LUCENE-3236]

PorterStemFilter has functionality to detect if a term has been marked as a "keyword" by the KeywordMarkerFilter (KeywordAttribute.isKeyword() == true), and if so, skip stemming.

The suggestion is to have the same functionality in other filters where it is applicable. I think it may be particularly applicable to the LowerCaseFilter (ie if it is a keyword, don't mess with the case), and StopFilter (if it is a keyword, then don't filter it out even if it looks like a stop word).

Backward compatibility is maintained (in both cases) by adding a new constructor which takes an additional boolean parameter ignoreKeyword. The current constructor will call this new constructor with ignoreKeyword = false.

Patches are attached (for LowerCaseFilter and StopFilter).

I have verified that the analysis JUnit tests run against the updated code, ie, backward compatibility is maintained.

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Migrated from [LUCENE-3236](https://issues.apache.org/jira/browse/LUCENE-3236) by Sujit Pal (@sujitpal), updated May 09 2016
Environment:
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Attachments: [lucene-3236-patch.diff](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3236/lucene-3236-patch.diff), [scan.pdf](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3236/scan.pdf)

Review LowerCaseFilter and StopFilter alongside the attached lucene-3236-patch.diff, then inspect the analysis JUnit tests mentioned in the issue. Confirm how KeywordAttribute.isKeyword() should affect each filter and preserve the current constructor behavior. Done means keyword-marked terms are handled as requested while existing analysis tests and backward compatibility remain intact.

Written by the indexing model from the issue text.