feat(accordion): add output for isOpen state changes

[steelsojka]

This fixes issue #2617 without making any breaking changes. This can solve several issues with accordion open state as well.

Closes #556

[codecov]

Codecov Report

Merging #2619 into development will increase coverage by 0.01%.
The diff coverage is 87.5%.

@@              Coverage Diff               @@
##           development   #2619      +/-   ##
==============================================
+ Coverage        70.69%   70.7%   +0.01%     
==============================================
  Files              137     137              
  Lines             4159    4165       +6     
  Branches           735     737       +2     
==============================================
+ Hits              2940    2945       +5     
  Misses             914     914              
- Partials           305     306       +1

Impacted Files	Coverage Δ
src/accordion/accordion-group.component.ts	90.32% <87.5%> (-1.68%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 479b046...4a7966a. Read the comment docs.

[IlyaSurmay]

@steelsojka hey, thanks for PR!
only one note, isOpenChanges should be renamed to isOpenChange
so you can use two way data binding

[steelsojka]

@IlyaSurmay I thought about that, but I thought it would be a breaking change since it would have broken one way bindings. I was thinking you would need to emit on isOpenChange instead of modifying the internal state, but you could modify the state and notify without breaking current behavior. I'll make the change.

[IlyaSurmay]

Tests are failing :( Please fix them, then we'll merge it.

[valorkin]

@steelsojka thanks for your contribution! ;)

[psined1]

Hi, any idea when is this going live?

[valorkin]

I am trying to publish small versions each Thursday, will be available as @next version
docs are here http://ngx-bootstrap.surge.sh/#/

[steelsojka]

@IlyaSurmay Thanks for fixing these tests and getting this working. I've been busy and wasn't able to get to them. I look forward to seeing this in a release!