Add the ability to hide a widget

[tlambert03]

Adds the ability to hide widget for a given parameter:

    @magicgui(call_button="call", viewer={"visible": False})
    def takes_viewer(viewer: napari.Viewer):
        ...

[codecov]

Codecov Report

Merging #17 into master will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   95.62%   95.69%   +0.07%     
==========================================
  Files           4        4              
  Lines         732      744      +12     
==========================================
+ Hits          700      712      +12     
  Misses         32       32              

Impacted Files	Coverage Δ
magicgui/_tests/test_magicgui.py	92.16% <100.00%> (+0.22%)	⬆️
magicgui/core.py	98.80% <100.00%> (+0.01%)	⬆️

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