add --emit-defaults option

[ktr0731]

Resolves #404.

Evans now supports --emit-defaults option which emits default values.

$ echo '{}' | ./evans -r cli call api.Example.Unary
{}

$ echo '{}' | ./evans -r cli call --emit-defaults api.Example.Unary
{
  "message": ""
}

[email protected]:50051> call Unary
name (TYPE_STRING) =>
{}

[email protected]:50051> call --emit-defaults Unary
name (TYPE_STRING) =>
{
  "message": ""
}

[codecov]

Codecov Report

Merging #410 (e00f4e4) into master (a6327f5) will increase coverage by 0.08%.
The diff coverage is 95.00%.

@@            Coverage Diff             @@
##           master     #410      +/-   ##
==========================================
+ Coverage   80.18%   80.26%   +0.08%     
==========================================
  Files          57       57              
  Lines        2740     2762      +22     
==========================================
+ Hits         2197     2217      +20     
- Misses        311      312       +1     
- Partials      232      233       +1