Implement `$.prototype.addBack`
Expose DOM level 1 API for Node-like objects

Now that htmlparser2's default DomHandler supports the `withDomLvl1`
option, enable it by default for all parsed content.

Update the project documentation and tests to use this standard API to
exemplify usage.
Add explicit tests for DOM level 1 API

Instead of implicitly relying on other unit tests to exhaustively assert
the "shape" of the Node-like object's API, define a single test to
ensure that the documented properties are available and functioning as
expected.
Merge pull request #561 from jugglinmike/dom-lvl-1-2

Expose DOM level 1 API for Node-like objects
Merge pull request #527 from jugglinmike/add-back

Implement `$.prototype.addBack`
fix lint error
Clone nodes according to original parsing options

Internally, Cheerio clones node structures by rendering them to a string
of HTML and subsequently parsing that string. Ensure that the
intermediary string is parsed according to the same options used to
create the target Cheerio object.
Merge pull request #563 from jugglinmike/551-clone-options

Clone nodes according to original parsing options
Implement `Cheerio#not`