41 functional rendering

[dfcook]

Addresses #41

1. Adds example of functional rendering using a render function.
2. Adds support for functional SFC's without a component definition by checking the isVueInstance property of the wrapper before calling destroy.

[codecov]

Codecov Report

Merging #82 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #82   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines          62     64    +2     
  Branches       10     11    +1     
=====================================
+ Hits           62     64    +2

Impacted Files	Coverage Δ
src/vue-testing-library.js	100% <100%> (ø)	⬆️

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[afontcu]

Oh, nice catch! 💯

(added a couple of inline suggestions to remove what appears to be some debugging boilerplate).