Add onClickItem event

[GreenRover]

I add the onClickItem event.
This is an requirement of one of my project.
where i have complex items in tree. And open onClick on the right halfe of the screen an bigger editing formular. There for is the href not very usefull.

You not need to add mentioned me as contributor.

[codecov]

Codecov Report

Merging #54 into master will decrease coverage by 0.34%.
The diff coverage is 36.36%.

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
- Coverage   40.98%   40.63%   -0.35%     
==========================================
  Files           9        9              
  Lines         405      438      +33     
  Branches       37       40       +3     
==========================================
+ Hits          166      178      +12     
- Misses        235      256      +21     
  Partials        4        4

Impacted Files	Coverage Δ
...x-tree-dnd-parent/ngx-tree-dnd-parent.component.ts	33.69% <33.33%> (-0.04%)	⬇️
...jects/ngx-tree-dnd/src/lib/ngx-tree-dnd.service.ts	35.6% <33.33%> (-0.24%)	⬇️
...ee-dnd-children/ngx-tree-dnd-children.component.ts	31.57% <50%> (+2.16%)	⬆️

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[GreenRover]

@Zicrael Hi is this pull request acceptable for you?

[andy008]

I've just included this code and it works nicely. Would be good to see it merged. Thank you @GreenRover :-)

[CrackerakiUA]

tested on npm i --save https://github.com/GreenRover/ngx-tree-dnd.git#dev-branch and on npm i --save https://github.com/GreenRover/ngx-tree-dnd.git. On both event is not fired

<lib-ngx-tree-component (onClickItem)="onClickItem($event)"></lib-ngx-tree-component>

@Zicrael this is really helpful feature, would be nice to have it under plugin. @GreenRover Could you also update readme?

[GreenRover]

@CrackerakiUA sorry dont like to invest in a dead project, please see how old this pr is