bug#13892: 24.3.50; Provide for customizing default regexp in hi-lock commands

[Juri Linkov]

> +(defvar hi-lock-read-regexp-defaults-function
> +  'hi-lock-read-regexp-defaults
> +  "Function that provides default regexp(s) for highlighting commands.
> +This function should take one argument OP and return one of nil,
> +a regexp or a list of regexps for use with highlighting command
> +OP.  OP, a symbol, can be one of `phrase', `line' or `nil'
> +signifying commands `hi-lock-face-phrase-buffer',
> +`hi-lock-line-face-buffer' and `hi-lock-face-buffer'
> +respectively.

Requiring an additional argument `op' means that we wouldn't
be able to customize `hi-lock-read-regexp-defaults-function'
to just `find-tag-default' or `find-tag-default-as-regexp'.
I think there is no need to distinguish between different
hi-lock commands since one user would very likely prefer
the one way to get the default for all hi-lock commands,
so you could call `hi-lock-read-regexp-defaults-function'
without arguments.