Allow using arbitrary expressions in confirmation prompt.

[casey]

Brought up in #1854. Should be easy enough! The parser should parse an expression instead of a string literal. One question is whether or not the expression should be evaluated with the recipe arguments in scope. An argument in favor of doing this is that it would allow giving more context on what exactly the recipe was doing, an argument against is that it requires evaluating recipe arguments, which you might not want to do unless you're going to run the recipe.