Added new method childrenCount complementary to child(int)

[literakl]

There is no easy way in the Element to find out a number of children available via the method child(int). The method childNodeSize() acts on list of all children while child(int) acts on the filtered list of the children. This trivial patch adds new method childrenCount() that is complementary to child(int) as it works on the same filtered list. A unit test is provided.

My observation: https://stackoverflow.com/questions/59547574/jsoup-how-to-find-out-elements-size

Thank you for this wonderful library and for consideration of merging this proposal.

[literakl]

It seems there is some configuration issue on github that the build fails:

The command "bash .install-jdk-travis.sh" failed and exited with 2 during .

[jhy]

Thanks! Merged with 894d3ec

I renamed it to be childrenSize for consistency with the other sizes of lists.