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.

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,20 +4,27 @@
 [docs/style-guide.md](../docs/style-guide.md)
 
 * commit message titles must be in the form:
+
 ```topic: Capitalized message with no trailing period```
+
 or:
+
 ```topic, topic2: Capitalized message with no trailing period```
 
 * golang code must be formatted according to the standard, please run:
+
 ```
 make gofmt		# formats the entire project correctly
 ```
+
 or format a single golang file correctly:
+
 ```
 gofmt -w yourcode.go
 ```
 
 * please rebase your patch against current git master:
+
 ```
 git checkout master
 git pull origin master
@@ -28,6 +35,7 @@ hub pull-request	# or submit with the github web ui
 ```
 
 * after a patch review, please ping @purpleidea so we know to re-review:
+
 ```
 # make changes based on reviews...
 git add -p		# add new changes