Added html method into Crawler class

* This method allow to get html inside desired element
* New method is tested
merged branch fabpot/moved_domcrawler_11 (PR #7750)

This PR was merged into the master branch.

Discussion
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Added html method into Crawler class

This PR was submitted on the symfony/DomCrawler read-only repository and moved automatically to the main Symfony repository (closes symfony/DomCrawler#11).

A pull request is waiting for aproval, but is not tested

* This method allow to get html inside desired element
* New method is tested

Commits
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a8d2e06 Added html method into Crawler class
[DomCrawler] finished the previous merge feature addition