Add new RSpec/Capybara/NegationMatcher cop

[ydah]

Resolve: #378

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Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

• Added the new cop to config/default.yml.
• The cop is configured as Enabled: pending in config/default.yml.
• The cop is configured as Enabled: true in .rubocop.yml.
• The cop documents examples of good and bad code.
• The tests assert both that bad code is reported and that good code is not reported.
• Set VersionAdded in default/config.yml to the next minor version.

If you have modified an existing cop's configuration options:

• [-] Set VersionChanged in config/default.yml to the next major version.

[Darhazer]

For spec completeness, can you add:
Case with heredoc for have_text (usually those are trickier for auto-correction, though we have some nice helpers to deal with those)

Test with argument, e.g. expect(page).not_to have_css('a') being corrected to expect(page).to have_no_css('a')

[pirj]

Looks good.
Thank you for another useful contribution, @ydah!
Small suggestions below.

Would the cop tolerate compound expectations like:

expect(page)
  .to have_no_css('foo')
  .and have_no_css('bar')

And does it even make sense to insist on using not_to for compound expectations? (Similar to I believe #1349).

[pirj]

Thank you, @ydah !