\w could match multibyte chars depending on system setup

Using the place holder @MEDIA(<mediaid>)@ you now can directly embed
images from the media repository into your HTML mail template.

An example was added to the mailwrap.html, but because images are
embedded as is (no prescaling) this is suboptimal. If we keep it, a
smaller version of the DokuWiki logo should be shipped with DokuWiki.

The mailwrap.html adds the standard headers and footers to all HTML
mails. The signature present in the text body will be removed before
inserting in the wrapper, allowing a nicer footer for HTML.

Users can overwrite the file with their own to create HTML mails in
corporate design. However, a way to automatically embed referenced
images is missing currently.

With this change it's easy to just set the replacements for HTML that
need special attention.

This method makes it easy to send a HTML mail based on the default text
mails. This is probably enough for simpler mails where no sophisticated
HTML is needed except a bit of formatting and linked URLs.

it now passes the whole Mail object and also signals if the mail sending
was successful to the AFTER event. A bunch of references should make it
compatible with old plugins.

this should make it easier to reintroduce a plugin hook compatible with
the old one

It's still not real world tested but the output *looks* right.

Plugin hook support is still missing.