markdown parser: allow setting target tag for links

markdown parser: allow setting target tag for links

If one really want's to force a link to open in a new tab (or window,
depending on the browser settings).

Note that we don't set target to _blank by default for links, as
opening in a new tab can already simply be done via a middle-click on
the link without that, but once the target is set opening in the same
tab cannot easily be done, i.e., without a target set the reader has
more freedom and flexibility.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>