Add simple autocorrect for Style/GuardClause

[FnControlOption]

This adds autocorrection for Style/GuardClause. For example:

def test
  if foo1
    return foo2
  else
    foo3
  end

  if bar1
    bar2
    bar3
  end
end

becomes

def test
  return foo2 if foo1

  foo3

  return unless bar1

  bar2
  bar3
end

More complex if-else statements are not autocorrected and left as-is:

if foo
  bar || return
else
  baz
end

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

The proposal looks reasonable to me. Might be a good idea to explain in the cop docs the scope of the auto-correct, so people do not have to wonder how it works exactly.

[dvandersluis]

@FnControlOption are you still interested in completing this PR?

[bbatsov]

@FnControlOption Please, rebase on top of master and squash your commits together.