Markdown lint (mdl) unclear output

[nitwof]

.overcommit.yml

PreCommit:
  Mdl:
    enabled: true

README.md

## test

Overcommit will fail with unclear message:

Running pre-commit hooks
Analyze markdown files with mdl.................................[Mdl] FAILED
Errors:
#<Overcommit::Hook::Message:0x00005637bca65c80>

mdl --json README.md will show:

[{"filename":"README.md","line":1,"rule":"MD002","aliases":["first-header-h1"],"description":"First header should be a top level header"}]

[nitwof]

I see the problem in parsing json response in mdl.rb:
https://github.com/sds/overcommit/blob/master/lib/overcommit/hook/pre_commit/mdl.rb#L22
JSON.parse returns a hash with string keys, but there is an access to hash by symbol.

I will fix it if you don't mind.

[sds]

Fixed in #678. Thanks!