Add chain from() and to() to contain mathcer

Support checking `contain()` matcher within the range specified in the file

e.g.

```sh
$ cat Gemfile 
 
group :test do
  gem 'rspec'
end
 
group :doc do
  gem 'rdoc'
end
```

```ruby
describe 'Gemfile' do
  it { should be_file }

  # check contain this file
  it { should contain 'rspec' }
  it { should contain 'rdoc' }

  # check contain ahead of 'group :test do ; end'
  it { should_not contain('rspec').to(/^group :test do/) }
  it { should_not contain('rdoc').to(/^group :test do/) }

  # check contain behind 'group :test do ; end'
  it { should contain('rspec').from(/^group :test do/) }
  it { should contain('rdoc').from(/^group :test do/) }

  # check contain within 'group :test do ; end'
  it { should contain('rspec').from(/^group :test do/).to(/^end/) }
  it { should_not contain('rdoc').from(/^group :test do/).to(/^end/) }

  # check contain within 'group :doc do ; end'
  it { should_not contain('rspec').from(/^group :doc do/).to(/^end/) }
  it { should contain('rdoc').from(/^group :doc do/).to(/^end/) }
end
```