Fix markup handling.

The current "allow_markup" setting will simply strip any markup from the final
notification, which includes formatting elements. On top of that, literal
[<>&..] symbols are not quoted before are being passed onto pango in several
places, resulting in stray error messages.

This patch fixes allow_markup to correctly strip markup only from the incoming
notification, not from the format.

You might also want to treat incoming messages as literal text (supplied by
un-aware programs), in which case you need to properly quote the text before
it's processed by pango. A new setting is introduced, called "plain_text",
which forces incoming messages to be treated literally.

allow_markup/plain_text are complimentary to each other.

The new rule actions allow to narrow down the handling to a specific block,
achieving notification Zen.

The following is done in this patch:

- Fix ruleset initialization in config.def.h.
- Introduce new allow_markup/plain_text actions in the rules.
- Fix handling of allow_markup to strip markup from summary/body only,
  preserving format's markup.
- Fix broken string functions (string_replace_all didn't handle recursive
  replacements correctly).
- Fix quoting of other literal fields (icon name/appname).
- Fix handling of ignore_newline as well (applied only on summary/body).
- Dunstrc update with the same previous defaults.