Added a method to close the preview

[christiaan-dockbite]

It is useful to close a preview programmatically. A method was added to support that.

[codecov[bot]]

Codecov Report

Merging #60 (5be3b73) into master (d2d8b9b) will decrease coverage by 0.04%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
- Coverage   34.86%   34.82%   -0.05%     
==========================================
  Files          16       16              
  Lines         806      807       +1     
  Branches      185      185              
==========================================
  Hits          281      281              
- Misses        486      487       +1     
  Partials       39       39              

Impacted Files	Coverage Δ
projects/gallery/src/lib/ngx-gallery.component.ts	34.40% <0.00%> (-0.28%)	:arrow_down:

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