[feature] Format string combinators

[ToxicFrog]

It is common in parser libraries to offer parser combinators, where the individual elements of the parser are values in their own right and combinator functions can be used to combine them into the complete parser; see, for example, LPEG. It would be nice if vstruct also had a combinator mode, where format elements (and format "strings") are first class values that can be conveniently combined. This ties in with #20 in that it provides a way to compose multiple format strings (or substrings).