Support contributors without github accounts

[chrisinajar]

What: Fix regression in which if you have any user already added who does not have a github account then the command line fails without any useful error messages.

Why: The spirit of all-contributors is such that all contributors get credit, not just those who are github savvy.

How: Test driven development

Checklist:

• Documentation -- N/A
• Tests
• Ready to be merged
• Added myself to contributors table

NOTE: This situation only arrises when you have manually added non-github users into your .all-contributorsrc file. This used to work but regressed in recent versions.

[codecov-io]

Codecov Report

Merging #77 into master will increase coverage by 1.2%.
The diff coverage is 85.71%.

@@            Coverage Diff            @@
##           master      #77     +/-   ##
=========================================
+ Coverage    51.6%   52.81%   +1.2%     
=========================================
  Files          18       18             
  Lines         312      320      +8     
  Branches       39       43      +4     
=========================================
+ Hits          161      169      +8     
+ Misses        136      134      -2     
- Partials       15       17      +2

Impacted Files	Coverage Δ
src/contributors/prompt.js	0% <0%> (ø)	⬆️
src/contributors/add.js	100% <100%> (ø)	⬆️
src/generate/format-contribution-type.js	100% <100%> (ø)	⬆️
src/generate/format-contributor.js	100% <100%> (ø)	⬆️

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[kentcdodds]

Super!

[kentcdodds]

Thank you so much :)