only set git user if one isn't already set

[hipstersmoothie]

What Changed

Previously auto would override the local git user with whatever was in the package.json author field.

Now:

• it first checks that one isn't set before configuring the email and name
• only tries to set the git user during changelog or shipit
• If a user is running npm version themselves (rather than using shipit) they are required to set the git config manually

Why

closes #124

Todo:

• Add tests (This file is still fully untested)
• Add docs
• Add SemVer label

[codecov]

Codecov Report

Merging #125 into master will decrease coverage by 0.84%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
- Coverage   75.02%   74.18%   -0.85%     
==========================================
  Files          14       14              
  Lines         973      976       +3     
  Branches      137      137              
==========================================
- Hits          730      724       -6     
- Misses        214      225      +11     
+ Partials       29       27       -2

Impacted Files	Coverage Δ
src/main.ts	20.28% <14.28%> (-3.17%)	⬇️

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[orta]

Thanks!