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.

TODO.md

@@ -1,4 +1,5 @@
 # TODO
+
 If you're looking for something to do, look here!
 Let us know if you're working on one of the items.
 If you'd like something to work on, ping @purpleidea and I'll create an issue
@@ -6,54 +7,70 @@ tailored especially for you! Just let me know your approximate golang skill
 level and how many hours you'd like to spend on the patch.
 
 ## Package resource
+
 - [ ] getfiles support on debian [bug](https://github.com/hughsie/PackageKit/issues/118)
 - [ ] directory info on fedora [bug](https://github.com/hughsie/PackageKit/issues/117)
 - [ ] dnf blocker [bug](https://github.com/hughsie/PackageKit/issues/110)
 
 ## File resource [bug](https://github.com/purpleidea/mgmt/issues/64) [:heart:](https://github.com/purpleidea/mgmt/labels/mgmtlove)
+
 - [ ] recurse limit support [:heart:](https://github.com/purpleidea/mgmt/labels/mgmtlove)
 - [ ] fanotify support [bug](https://github.com/go-fsnotify/fsnotify/issues/114)
 
 ## Svc resource
+
 - [ ] base resource improvements
 
 ## Exec resource
+
 - [ ] base resource improvements
 
 ## Timer resource
+
 - [ ] increment algorithm (linear, exponential, etc...) [:heart:](https://github.com/purpleidea/mgmt/labels/mgmtlove)
 
 ## User/Group resource
+
 - [ ] automatic edges to file resource [:heart:](https://github.com/purpleidea/mgmt/labels/mgmtlove)
 
 ## Virt (libvirt) resource
+
 - [ ] base resource improvements [:heart:](https://github.com/purpleidea/mgmt/labels/mgmtlove)
 
 ## Net (systemd-networkd) resource
+
 - [ ] base resource
 
 ## Nspawn (systemd-nspawn) resource
+
 - [ ] base resource [:heart:](https://github.com/purpleidea/mgmt/labels/mgmtlove)
 
 ## Mount (systemd-mount) resource
+
 - [ ] base resource [:heart:](https://github.com/purpleidea/mgmt/labels/mgmtlove)
 
 ## Cron (systemd-timer) resource
+
 - [ ] base resource [:heart:](https://github.com/purpleidea/mgmt/labels/mgmtlove)
 
 ## Http resource
+
 - [ ] base resource [:heart:](https://github.com/purpleidea/mgmt/labels/mgmtlove)
 
 ## Etcd improvements
+
 - [ ] fix embedded etcd master race
 
 ## Torrent/dht file transfer
+
 - [ ] base plumbing
 
 ## GPG/Auth improvements
+
 - [ ] base plumbing
 
 ## Language improvements
+
 - [ ] more core functions
 - [ ] automatic language formatter, ala `gofmt`
 - [ ] gedit/gnome-builder/gtksourceview syntax highlighting
@@ -61,6 +78,7 @@ level and how many hours you'd like to spend on the patch.
 - [ ] emacs syntax highlighting
 
 ## Other
+
 - [ ] better error/retry handling
 - [ ] deb package target in Makefile
 - [ ] reproducible builds