Release 0.1.11-beta-1: Language improvements and fixes

This beta release adds one language extension, three functions, and
fixes one bug.

When creating objects for output the key can now be computed with a
JSLT expression.

Three functions have been added:

• zip-with-index
• zip
• index-of

And, finally, the JSLT implementation has been made fully thread-safe
by making the regular expression cache thread-safe.