(maint) Add markdown lint checking to markdown tests

[glennsarti]

Previously the markdown generated by Puppet-Strings may contain errors which
markdown lint programs would flag. This commit adds the ability to run markdown
lint on a per test basis and adds pending tests to show that the markdown output
does indeed need some work.

Note that this commit does not fix the errors. That will come in later commits.

[glennsarti]

e.g.

Pending: (Failures listed here are expected and do not affect your suite's status)
  1) PuppetStrings::Markdown rendering markdown to a file with common Puppet and ruby content should not generate markdown lint errors from the rendered markdown
     # Failures are expected
     Failure/Error: expect(File.read(file.path)).to have_no_markdown_lint_errors
     
       expected that "# Reference\n<!-- DO NOT EDIT: This document was generated by Puppet Strings -->\n"... would have no markdown lint errors but got output.md:236: MD013 Line length
       output.md:237: MD013 Line length
       output.md:326: MD013 Line length
       output.md:436: MD013 Line length
       output.md:1: MD022 Headers should be surrounded by blank lines
       output.md:128: MD024 Multiple headers with the same content
       output.md:139: MD024 Multiple headers with the same content
       output.md:143: MD024 Multiple headers with the same content
       output.md:151: MD024 Multiple headers with the same content
       output.md:162: MD024 Multiple headers with the same content
       output.md:170: MD024 Multiple headers with the same content
       output.md:202: MD024 Multiple headers with the same content
       output.md:228: MD024 Multiple headers with the same content
       output.md:245: MD024 Multiple headers with the same content
       output.md:247: MD024 Multiple headers with the same content
       output.md:255: MD024 Multiple headers with the same content
       output.md:259: MD024 Multiple headers with the same content
       output.md:286: MD024 Multiple headers with the same content
       output.md:318: MD024 Multiple headers with the same content
       output.md:335: MD024 Multiple headers with the same content
       output.md:337: MD024 Multiple headers with the same content
       output.md:343: MD024 Multiple headers with the same content
       output.md:349: MD024 Multiple headers with the same content
       output.md:355: MD024 Multiple headers with the same content
       output.md:365: MD024 Multiple headers with the same content
       output.md:382: MD024 Multiple headers with the same content
       output.md:396: MD024 Multiple headers with the same content
       output.md:398: MD024 Multiple headers with the same content
       output.md:404: MD024 Multiple headers with the same content
       output.md:410: MD024 Multiple headers with the same content
       output.md:422: MD024 Multiple headers with the same content
       output.md:442: MD024 Multiple headers with the same content
       output.md:450: MD024 Multiple headers with the same content
       output.md:456: MD024 Multiple headers with the same content
       output.md:467: MD024 Multiple headers with the same content
       output.md:473: MD024 Multiple headers with the same content
       output.md:490: MD024 Multiple headers with the same content
       output.md:522: MD024 Multiple headers with the same content

[glennsarti]

CI failures are due to #222