Allow setting default delimiters

[Thomas Kappler]

Feature request from I. B.: "Might you be able to create a variable to hold the default value for the beginning and end delimiters? That way if you're in say, java mode you can set the default to { and } so you can just hit return when prompted for the delimeters, whereas if you're in say f90-mode (Fortran) you can set these (via a buffer local variable, etc.) to (, )."

We probably one string variable a la "()" instead of a "from" and a "to" variable.