rpl-mode: electricity and customization

Add electricity to plain keystrokes that make balanced things on the
calculator, instead of having to C-c them.  Also, make it customizable.

Fix fontification of \:- so that it isn't an error.

rpl-pretty-trigraphs still does nothing when set nil.  Low priority.
rpl-mode: more thorough font locking

Get more keywords fontified.  Have DOERR be a warning.  SYSEVAL and
PURGE are marked specially.
rpl-mode: font lock HOME and obvious variables

HOME is a known global variable.

System names, according to the user manual, never start with lowercase
letters.  Always fontify these as variables.
rpl-mode: compiled local variables

A name that starts with \<- is a compiled local variable, always.