adds support for FirstName of either male or female gender

[perryao]

I need to generate a random first name of either gender, so added support for that here. From a user perspective, if they choose to define a struct with either TitleMale or TitleFemale, they should also use FirstNameMale or FirstNameFemale.

But if they only need a FirstName and a LastName, this makes it possible to generate Persons of random gender.

[codecov]

Codecov Report

Merging #28 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   97.82%   97.83%   +0.01%     
==========================================
  Files           8        8              
  Lines         368      370       +2     
==========================================
+ Hits          360      362       +2     
  Misses          4        4              
  Partials        4        4

Impacted Files	Coverage Δ
faker.go	95.23% <ø> (ø)	⬆️
person.go	100% <100%> (ø)	⬆️

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[bxcodec]

LGTM