Add num3-mode
num3-mode: use “num3--” as prefix for private symbols

Previous version of num3-mode used “-num3” as a prefix for private
symbols which goes against the style used in the rest of Emacs.
This commit changes the prefix to “num3--”.
* num3-mode.el: Fix footer.  Reported by Jonas Bernoulli <jonas@bernoul.li>.
* num3-mode.el: Catch strings looking like hex number even w/o 0x prefix.

Update `num3--number-re' to catch strings which are a whole word, have
only hexadecimal digits and have at least one non-decimal hexadecimal
digit (i.e. letter from A to F), and treat them as hexadecimal number.

Also update `num3--number-re' to properly catch Emacs Lisp's

This fixes bug#16834.
* packages/num3-mode/num3-mode.el (num3-mode): Use font-lock-flush
if available.