Setting position and offset gaps in IndexableField [LUCENE-5073]

I often find it to be quite the hassle to go via Analyzer in order to add offset/position gaps when adding data the the same field name.

This patch introduce IndexableField#positionIncrementGap and #offsetGap, which at index time is added to the value gathered from the correlating Analyzer#methods.

No tests in here, yet, but first a question: am I missing something important in the design of Lucene here? Why is this not already a feature?

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Migrated from [LUCENE-5073](https://issues.apache.org/jira/browse/LUCENE-5073) by Karl Wettin
Attachments: [LUCENE-5073.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-5073/LUCENE-5073.patch)

Start with the IndexableField and Analyzer APIs referenced in the issue, then review the attached LUCENE-5073.patch. Resolve whether field-level positionIncrementGap and offsetGap fit Lucene's indexing design; done means the design is agreed and the resulting behavior has regression tests.

Written by the indexing model from the issue text.