Markdown lint hook fails with unexpected output

[steinybot]

With the following .overcommit.yml:

PreCommit:
  Mdl:
    enabled: true

And a README.md:

## Oops

Then attempting to commit will fail with:

Analyze markdown files with mdl.................................[Mdl] FAILED
Unexpected output: unable to determine line number or type of error/warning for output:

A detailed description of the rules is available at https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md

Running mdl README.md manually will show:

README2.md:1: MD002 First header should be a top level header

A detailed description of the rules is available at https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md

I guess this extra line in the output may be new in mdl and has broken the hook.

[sds]

Looks like mdl will always display that message unless you use JSON output:
https://github.com/markdownlint/markdownlint/blob/104bd8591b193437801d2a0d17b47a60fdc129e6/lib/mdl.rb#L108-L113

Would happily merge a pull request updating the hook use the JSON format. Thanks!