Include HTML source in report

[sveetch]

Alike the online validator, report should contain the full checked HTML source so user can look at it for validation message since the truncated extract part in each message is not always helpful.

Unfortunately the vnu validator does not seems to have any option to include the source in its report, it seems to do so only on its frontend.

This will force us to change the way we use the validator. Instead of just giving path/urls to its CLI, we will need to do some trick like getting URL source with requests lib, write it to a temporary file which will be given to the validator and transport the source in report. Path files will be more simple since we will just need to read it to get the source.

This is a major breaking change in the API since it will break many tests.