Add basic govuk_link_to helper and test

[peteryates]

This is a bit of an experiment related to the suggestion made in the comments of #3, it adds a app/helpers directory and an initial #govuk_link_to helper that's a wrapper of Rails' #link_to

• Add a proof of concept helper

• Ensure it works with the dummy app
  

• Add a test (and make it pass!)

Testing currently doesn't work

• in the submitted state we get undefined method link_to'`
• adding include ActionView::Helpers::UrlHelper to the top of govuk_link_helper_spec.rb or spec_helper.rb makes the individual file pass but when running the suite we get cyclic include detected

Not quite sure how to make it play ball with RSpec.

[rjlynch]

👍 Looks good to me

[aliuk2012]

Can you pass custom css classes?

[peteryates]

Just finishing off that functionality @aliuk2012, ensuring it works for strings and arrays

[peteryates]

@aliuk2012 so yeah, having had a think any custom classes will override the default. Ensuring safe merging made it a bit too complex. We can revisit this behaviour if needs be