Allow custom suffixes for resources files

[alexiscreuzot]

Current state

Currently, resources files are managed using this function :
https://github.com/thoughtbot/liftoff/blob/master/lib/liftoff/project_builder.rb#L90-L92

Problem

In some projects, the user might want to use custom templates files for resources, like .json, .txt or .yml files, without having to manually remove them from the "compile sources" build phase afterward.

Proposed solution

Adding an optional array to the liftoffrc file named additional_resources, used likeso :

additional_resources:
 - txt
 - json
 - yml

This array could then be added to the defaults suffixes in the project_builder.rb resource_file? function.

[alexiscreuzot]

Any update on my PR?

[gfontenot]

hey @kirualex, really sorry for the radio silence. I'm being a poor maintainer. This PR made me start to think of some larger things and so I haven't had a chance to give it the attention I'd like. I'm going to try to go through it on Friday or some time next week. Thanks for your patience!

[Maximelc]

I use this method for custom extension #277