[Feature] index word can be differ with head word

[miurahr]

Is your feature request related to a problem? Please describe.

Some dictionary has head word entry like "zonbi(e)" that means index word is "zombi" and presentation head word is "zombie".
Current DSL4j indexed "zombi(e)" so it does not hit search word "zombie"

Describe the solution you'd like
Extend internal index data from (headWord, {articleOffset, articleSize}) to (indexWord, {headWordsOffset, headWordsSize}, {articleOffset, articleSize})
Search should hit with index word, and return headwords and article as a result.

Describe alternatives you've considered
We can store head words in the index data but it might require more memory to save.

Additional context
Index cache definition should be extended to store head word index.