Add ability to specify output filename

[felipemfp]

Refactor default filename to snake_case, because useraddresssliceloader_gen.go is really unreadable.

Lemme know your thoughts on this one.

BTW, thanks you!

[codecov]

Codecov Report

Merging #32 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #32   +/-   ##
=======================================
  Coverage   85.57%   85.57%           
=======================================
  Files           2        2           
  Lines         104      104           
=======================================
  Hits           89       89           
  Misses         15       15

Impacted Files	Coverage Δ
example/user_loader_gen.go	94.68% <ø> (ø)

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[blessedvictim]

one note from my side.
i think don't need use strcase library. no need to add an extra dependency. What you think?