Add recipe checker command and script

* Add el-get-check-recipe command
* Add a script test/check-recipe.el
* Mention recipe checker in the document
* Add warnings to el-get-check-recipe
* Use @command instead of @code
* make el-get.info
* Use more expressive name than el-get-check-recipe-1
Merge pull request #1012 from tkf/recipe-checker

Add recipe checker command and script