Add click functionality

[n3wtron]

Added click functionality.
Created 2 functions:
now the component accepts 1 new input parameter isSegmentClickable and emits a new signal segmentClicked.
The first one is an input function that returns a boolean

function isClickable(segment:NgxJsonSegment):boolean

the signal can be used to handle the click event

function segmentClickHandler(segment:NgxJsonSegment)

[MichaelDoyle]

This is an old PR, but just in case anyone is looking for this, I needed configurable "click handling" for my own work, and ended up forking the project and modernizing it.

https://github.com/MichaelDoyle/ngx-json-treeview