#45 Custom prompt

[Jepson]

[Suggested by Lumsby]

A custom prompt, plus standard elements to be inserted
as the user wants.

The prompt string must be simple enough for the user to
edit using the standard line editor. Help texts should be
provided.

Examples of elements might include date parts, time
parts, next command, current conference, total unread
texts, unread texts in current conference and so on.

I suggest writing a prompt formatter, adding a prompt
format field to the user table (unless we take this
opportunity to implement property bags) and giving all
current (and new) users a standard prompt.