nvim: implements neovim/neovim@4ae31c4 APIs

[zchee]

Implements neovim/neovim@4ae31c4 APIs.

[codecov]

Codecov Report

Merging #84 (20b6209) into master (3d2a7d9) will increase coverage by 0.2%.
The diff coverage is 37.5%.

@@           Coverage Diff            @@
##           master     #84     +/-   ##
========================================
+ Coverage    57.4%   57.6%   +0.2%     
========================================
  Files          11      11             
  Lines        2124    2132      +8     
========================================
+ Hits         1221    1230      +9     
+ Misses        769     768      -1     
  Partials      134     134             

Flag	Coverage Δ
Linux-1.13.x	57.6% <37.5%> (+0.2%)	⬆️
Linux-1.14.x	57.6% <37.5%> (+0.2%)	⬆️
Linux-1.15.x	57.6% <37.5%> (+0.2%)	⬆️
macOS-1.13.x	57.6% <37.5%> (+0.2%)	⬆️
macOS-1.14.x	57.6% <37.5%> (+0.2%)	⬆️
macOS-1.15.x	57.6% <37.5%> (+0.2%)	⬆️

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Impacted Files	Coverage Δ
nvim/apiimp.go	25.8% <37.5%> (+1.6%)	⬆️

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[zchee]

I'll write more test case on #82