Add new options to Command resource (goss-org#434)

* Add optional exec attribute to command definitions
Allow skip attribute in command definitions

Documentation changes for new command exec attribute, and skip attribute

Update CI tests to match changes to the command exec attribute

* Disable dnstest (goss-org#440)

* Disable dnstest.io tests, seems to be down

* Update md5 for latest docker images

* Add tests for command and skip
Fix problem with skipping first test
Fix where commands add exec attribute automatically

* gofmt loves tabs and vertical space

docs/manual.md

@@ -343,7 +343,7 @@ service:
 file:
   /var/www/html:
     filetype: directory
-    exists: true  
+    exists: true
 ```
 
 If you try to validate this file, it will **only** run the second `file` test:
@@ -371,7 +371,7 @@ file:
     exists: true
   /var/www/html:
     filetype: directory
-    exists: true  
+    exists: true
 
 service:
   httpd:
@@ -440,7 +440,7 @@ command:
     # required attributes
     exit-status: 0
     # defaults to hash key
-    exec: "go version" 
+    exec: "go version"
     # optional attributes
     stdout:
     - go version go1.6 linux/amd64