Add ability to give berkshelf an additional path for cookbook templates via config or cli 

I'm not sure if this is the best way to do this, but it is A way.  If this is sensible I can look at adding tests.  If not, feedback pointing me in the right direction would be ace.

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Add a --cookbook_templates cli and config file option to provide an extra path to search for cookbook templates.

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Addresses RiotGames/berkshelf#437