If you just need formatting without logic, then you could use something like Markdown.
Someone else already mentioned GRMustache [1], and that looks pretty good.
I'm not sure if there is an iOS version, but you could look for something like liquid [2]. I think liquid is usually a better choice for user-controlled templates, especially when you're rendering on a server. But if they're just compiling the templates on their own device, then handlebars (GRMustache) would be the right way to go.
Someone else already mentioned GRMustache [1], and that looks pretty good.
I'm not sure if there is an iOS version, but you could look for something like liquid [2]. I think liquid is usually a better choice for user-controlled templates, especially when you're rendering on a server. But if they're just compiling the templates on their own device, then handlebars (GRMustache) would be the right way to go.
[1] https://github.com/groue/GRMustache
[2] https://github.com/Shopify/liquid