test, docs: Add a linter for testing markdown, and fix up our docs

While writing docs, I couldn't remember what the correct style was
supposed to be, and I remember someone complaining about this
previously, so I decided to add a linter! I excluded a bunch of annoying
style rules, but if we find more we can add those to the list too.

Hopefully this gives us a more consistent feel throughout.

docs/puppet-guide.md

@@ -109,8 +109,8 @@ file { "/tmp/mgmt-test":
 
 To avoid this, specify the parameter explicitly:
 
-```
-$ puppet mgmtgraph print --code 'file { "/tmp/mgmt-test": backup => false }'
+```bash
+puppet mgmtgraph print --code 'file { "/tmp/mgmt-test": backup => false }'
 ```
 
 This is tedious in a more complex manifest. A good simplification is the