A start at a gem-based plugin whitelist for Pages.
Add --whitelist flag and internal logic
Remove Site.whitelist attribute.
Add test code for 'whitelist' option.
Remove option from CLI
Add "jekyll_test_plugin_malicious" as gem dep
The whitelist should be an array (not a file)

Default to `[]` if the key’s value is falsey
Hm, shouldn't need that call to 'rescue' in Site#whitelist
Refactor conditions for allowing plugins into a new method: Site#plguin_allowed?
Print the output of Jekyll if the command fails
Whoops, it's called 'jekyll_run_output'
I mean the argument and the local variable should be the same thing
Merge pull request #1657 from mojombo/safe-whitelist

A gem-based plugin whitelist for `safe` mode