Add the jni method `getContentPart` to get only a part of the artcicle
content.
The method can be used to get a part of the content or to know the size
of the full content.
This is a major API break. User code will have to be rewritten.
Before this commit, API was a unique object wrapping the library and
handle a global state with one `Reader` and one `Writer` at the time.
Now, the API is axed around three main objects :
- The `JNIKiwixReader`, a wrapper around a `kiwix::Reader` (who allow to
read one zim)
- The `JNIKiwixSearcher`, a wrapper around a `kiwix::Searcher` (who allow
to search through one or more reader(s))
- The `JNIKiwixSearcher.Result` a result of a search. Allowing to get all
information about a result (title, url, content, snippet, ...)