Allow omission of observed keys and handling of cyclic references

[BennyH26]

When observing large objects, for efficiency it would be very useful to allow omission of certain observed keys. This was originally brought to my attention because an object I was observing contained a cyclic reference (and thus omitting it would have saved the day). In the same vein, in addition to omitting certain keys, it would be useful to allow object-observer to handle cyclic references by only observing the first instance of such keys.

[gullerya]

Please take a look onto #129 specifically for the circular piece, any remarks on the said there are welcome.

[gullerya]

The circular thing handled as part of #129 and released in 5.1.7.
The optional skip paths or other kind of approach for optioning the creation of Observable will be considered as part of the next major 6.x.x version (which I'm moving to be published as a scoped package @gullerya/object-observer), please follow up that one.