Add more examples (chaining, self-reference, and choices)

[tlambert03]

Just adding a few more examples to the repo:

chaining shows two different ways to chain the output of one function into the input of another function, and autocall (only what is necessary) upon change. (cc @haesleinhuepf)

self-reference shows that you can update the parameters of the widget itself from within the decorated function (thanks for the inspiration on that one @haesleinhuepf! :))

choices shows all of the valid ways to provide a dropdown. (cc @haesleinhuepf)

[codecov]

Codecov Report

Merging #85 (ad12102) into master (f119363) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #85   +/-   ##
=======================================
  Coverage   87.77%   87.77%           
=======================================
  Files          15       15           
  Lines        1619     1619           
=======================================
  Hits         1421     1421           
  Misses        198      198           

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