FIX: #6
Introducing options that control the parsing behavior.

Changes the default behavior to be lenient towards uplicate-key occurances, but allows a strict mode that brings back the old behavior.

Includes test and documentation on usage.

Caveat: we should consider a significant version-bump when releasing this, since it
* changes default behavior
* extends/changes the api
Redone Fix: #6

Inverting logic of options.lenient to options.strict since it seems more self-explaining.
Moving the (options) passing to the instantiation rather then the specifc parse.
Updating tests and docs

Proposing a "big enough" version update to clearly indicate the incompatible API changes in this fix.
Make the fix for #71 backwards compatible so we can publish this fix with "only" a minor version-number increase

As per suggestions of @sidorares in
* https://github.com/marc-portier/json-bigint/commit/04723e0a9023f93b677f37c4a708288c07d63fcb#commitcomment-7397586 and
* https://github.com/marc-portier/json-bigint/commit/04723e0a9023f93b677f37c4a708288c07d63fcb#commitcomment-7397623
important backwards-compat change: this remnant of earlier sollutions must be removed
Merge pull request #7 from marc-portier/master.patch#6

Patch for #6 - introducing the option to parse "strict"  or lenient.