Slug regex

Prevent multiple consecutive dashes in slug - I'm guessing from the comment in pattern.rb that this is what was intended?
Whilst this does test what the behaviour that you've added, I think we'd be better off with a lower-level test. I tried testing the model, but that quickly became clunky. I think a unit test of the pattern probably works best.

Would you be able to undo the last commit and instead add a file in `spec/lib/valid_slug_spec.rb` containing:
```ruby
require 'rails_helper'
require 'valid_slug/pattern'

describe ValidSlug::PATTERN do
  it 'matches good slugs' do
    expect('a-to-z').to match(ValidSlug::PATTERN)
  end

  it 'does not match slugs with uppercase characters' do
    expect('aTOz').to_not match(ValidSlug::PATTERN)
  end

  it 'does not match slugs with leading dashes' do
    expect('-atoz').to_not match(ValidSlug::PATTERN)
  end

  it 'does not match slugs with leading dashes' do
    expect('atoz-').to_not match(ValidSlug::PATTERN)
  end

  it 'does not match slugs with underscores' do
    expect('a_toz').to_not match(ValidSlug::PATTERN)
  end

  it 'does not match slugs with consecutive dashes' do
    expect('a--toz').to_not match(ValidSlug::PATTERN)
  end
end
```