Add MarkdownLint CLI checker

[dholm]

Adds support for the MarkdownLint Command Line Interface based on the
Node.js markdownlint package.

• https://github.com/igorshubovych/markdownlint-cli

[CLAassistant]

All committers have signed the CLA.

[cpitclaudel]

Looks very good :) Thanks! Could you add an entry in CHANGES?

[dholm]

@cpitclaudel Done!

[cpitclaudel]

Thanks! One last thing: you should use .. syntax-checker-config-file instead of .. defcustom for the config file (that's what causing the build to fail)

[dholm]

Oops, fixed!

[fmdkdd]

I'm not sure it was necessary to restyle every headings. But I guess it's your mode that made the change sneakily?

[dholm]

Hmm, yes, I didn't even catch that myself.

[dholm]

Fixed

[fmdkdd]

Great! We just need you to squash your commits before we can merge :)

[dholm]

@fmdkdd Memo to self, never edit code on GitHub. :/

[fmdkdd]

@dholm Thanks. One last thing: how does markdown-cli compare to mdl, the other linter we already have in Flycheck? I want to know if it's worth changing the default to markdown-cli, or whether we should keep mdl (because it has more lints/has better lints/is more performant/more popular or whatever).

[dholm]

@fmdkdd I used mdl at first but had issues with suppressions not working according to the documentation, for instance disabling the line-length check for tables. markdownlint-cli appears to be better maintained and did not have these issues.

[fmdkdd]

@dholm I see, thank you.

@marsam You added mdl, do you have an opinion on whether we should switch to markdownlint-cli as the default checker for Markdown?

[cpitclaudel]

Ping @marsam , any opinions on changing the default checker? :)

[marsam]

Sorry for the late response, I don't have any issues with changing markdownlint-cli as default checker

[cpitclaudel]

@marsam Merged, thanks @dholm! (And sorry for the delay) Let's change the default checker in a separate PR. @dholm, would you like to open one?

[dholm]

@cpitclaudel Thank you! Isn't it already the default though? I thought the order in flycheck-checkers is what makes it default.