Add printfilename option

[di]

This PR adds the printfilename flag, which includes the filename which produced the error in the resulting error message (a la https://www.gnu.org/prep/standards/html_node/Errors.html)

This is useful when using html-linter with multiple input files.

For example:

$ html_lint.py --printfilename test/data/more_invalid.html
test/data/more_invalid.html:2:4: Error: Document Type: Do not close void elements: Remove the trailing "/" from the br tag.

[coveralls]

Coverage increased (+0.003%) to 99.471% when pulling 9b3a937 on di:add-printfilename into ab6ac07 on sk-:master.

[di]

@sk- Any chance on this getting merged?

[alanhdu]

FWIW, I would love to see this PR merged and released. It's a life-saver for linting dozens of files at once. Do you think that's possible @sk-?

[sk-]

Thanks for the PR, sorry for the delay.

[di]

Thanks @sk-. Are you planning a release anytime soon?

If this package needs an additional maintainer, I'd be happy to help.

[sk-]

@di Dustin, that would be awesome. I sent you an invite to be a collaborator on the project. Could you send me your Pypi username so I can add you as a maintainer of the package.

For travis I think it should be automatic once you have collaborator access to the project.

[di]

@sk- Awesome, thanks! My username is the same: https://pypi.org/user/di/

[di]

@alanhdu This has been released: https://pypi.org/project/html-linter/0.4.0/