Split Candidates#each in few methods

[novikserg]

Hey,

This PR splits Candidates#each method, thus separating actual yielding from constructing/filtering elements.

A minor change, but we can further improve from here: one idea is to create a Candidate class, and move slug construction/validation there.

Should also solve CodeClimate Issue (https://codeclimate.com/github/norman/friendly_id/FriendlyId::Candidates#complexity)

Main idea is to make this part of code more readable (while keeping simplicity/flexibility)

[coveralls]

Coverage decreased (-0.4%) to 96.296% when pulling 315af6f on novikserg:feature/refactor_candidates_each into aff0564 on norman:master.

[coveralls]

Coverage decreased (-0.4%) to 96.296% when pulling f0a819a on novikserg:feature/refactor_candidates_each into aff0564 on norman:master.

[parndt]

Thanks @novikserg