Make the “tabbed” resource prefix configurable.

This adds a tabbedex-rs-prefix resource which, when set, changes the
prefix tabbedex will use for the resource names.  In particular,

    URXvt.tabbed.tabbedex-rs-prefix: tabbedex

will cause the plugin to read all the other resources from
“URXvt.tabbedex.*” namespace rather then “URXvt.tabbed.*”.

This commit was originally coded by Mark Pustjens <mark@unkie.org>,
but it hard-coded the change thus breaking backwards compatibility.
My version keeps the code behaving as before unless explicit
configuration change on user part is made.
New blacklist resource  preventing extensions from loading in spawned tabs.

Needed when using kuake in combination with tabbedex.

[mina86@mina86.com: Conflicts resolved, changelog entry added.]