cmd/ng-gengo: add a command to generate a Go package from a Ng script

[sbinet]

Command ng-gengo generates a Go package file from a Neugram script.

    Usage: ng-gengo [options] file1.ng

    ex:
     $> ng-gengo ./eval/testdata/defer1.ng
     $> ng-gengo ./eval/testdata/defer1.ng > defer1.go
     $> ng-gengo -pkg=main ./eval/testdata/defer1.ng

    options:
      -pkg string
            name of the output Go package (default "main")

[codecov-io]

Codecov Report

Merging #198 into master will decrease coverage by 0.13%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
- Coverage   49.99%   49.85%   -0.14%     
==========================================
  Files          22       22              
  Lines       10406    10406              
==========================================
- Hits         5202     5188      -14     
- Misses       4702     4714      +12     
- Partials      502      504       +2

Impacted Files	Coverage Δ
parser/scanner.go	68.92% <0%> (-2.2%)	:arrow_down:
parser/parser.go	70.05% <0%> (-0.22%)	:arrow_down:

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[sbinet]

ah... I didn't notice that :)

I created this little command mainly for debugging purposes (ie: fixing the gengo tests when implementing a new thing under eval/testdata)

I can drop this if you want (the ng -o main foo.ng modus operandi suits me.)

[crawshaw]

Submitting it for debugging is fine with me. Maybe mention that in its docs somewhere?