Initial support for labels

[tlambert03]

This PR adds labels (using the argument name) by default. Can be turned off with @magicgui(labels=False)... works with both @magicgui(layout='horizontal') and 'vertical'

[codecov]

Codecov Report

Merging #14 into master will decrease coverage by 0.08%.
The diff coverage is 88.46%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
- Coverage   95.71%   95.62%   -0.09%     
==========================================
  Files           4        4              
  Lines         700      732      +32     
==========================================
+ Hits          670      700      +30     
- Misses         30       32       +2     

Impacted Files	Coverage Δ
magicgui/_qt.py	96.94% <85.18%> (-1.24%)	⬇️
magicgui/_tests/test_magicgui.py	91.93% <91.66%> (+0.46%)	⬆️
magicgui/core.py	98.79% <92.30%> (-0.39%)	⬇️

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[jni]

😍