Add option for label format string

[aspiers]

Rather than just replacing the address with the label string, it could be replaced by a string generated from a format string specified as a user option. For example, if the format string was %s | 0x%6l...%4r then the address would be replaced by something like my wallet address | 0x123456...1234.

Potential substitution characters

• %s -> the whole label string
• %Ns -> label string, limited to at most the first N characters
• %Nl -> left-most N characters of the address (excluding the initial 0x, to allow the user to omit that if they want)
• %Nr -> right-most N characters of the address