Allow expressions as arguments to attributes

[casey]

Currently, recipe attributes like [confirm: 'prompt'] only allow string literal arguments. Ideally, these should all be able to take arbitrary expressions, like [confirm: 'are you sure you want to delete ' + path + '?'].

This has been brought up in a number of issues:

#553
#1855
#1996
#2510
#2521

[casey]

Looking at the attributes, I think all of them should be able to take expressions as arguments, with the possible exception of [group], which needs to be evaluated without executing anything.