Allow specifying _pre-requisites_ within templates

[fervic]

@aspring, I was wondering if it makes sense to add the ability of specifying which template needs to be processed before a template but within the template itself.

So that if you want to merge multiple templates like in the example:

$ racker rackertemplate1.rb rackertemplate2.rb rackertemplate3.rb packertemplate.json

You just have to specify the last one:

$ racker rackertemplate3.rb packertemplate.json

Because it already knows that depends on template 1 & 2. Or it knows that depends on template 2 and template 2 knows it depends on template 1.

Maybe a simple require_relative would do but it would be nicer if it was something more integrated into the DSL.