Shellinford

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- Make "in" operator faster

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- `FMIndex.count()` is added
- No longer support Python 2.6
- bug fix

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- `FMIndex.build()` and `FMIndex.pushback()` ignore empty string
- `FMIndex` supports "in" operator. (e.g., 'a' in fm)
- Support Python 3.5, 3.6 and 3.7

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- `FMIndex.search()` returns list

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- "OR" search and "NOT" search are available in `FMIndex.search()`.
- `FMIndex.size` and `FMIndex.docsize` are available as property

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"AND" search is available by giving Sequence (list, tuple, etc.) `FMIndex.search()`