make w3 validator-url configurable

[strowi]

Hi,

just a suggestion: it would be very handy to change the w3-validator to a self-hosted instance.

regards, strowi

[dpalic]

Thank you for the issue report. Sadly this project is dead, and a new team is around with https://github.com/linkcheck/linkchecker for more details please see: #708 Also please close this issue and report it freshly on the new repo https://github.com/linkcheck/linkchecker/issues if your issue still persists