Add window argument to electrondisplay

[tkf]

This PR adds new signatures electrondisplay(window, [mime,] x) to electrondisplay. It lets you display things in pre-existing Electron.Window.

[codecov-io]

Codecov Report

Merging #41 into master will increase coverage by 0.61%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   82.14%   82.75%   +0.61%     
==========================================
  Files           2        2              
  Lines         112      116       +4     
==========================================
+ Hits           92       96       +4     
  Misses         20       20

Impacted Files	Coverage Δ
src/ElectronDisplay.jl	78.26% <90%> (+0.98%)	⬆️

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