Making xtl documentation available through xeus-cling's quick help feature

[SylvainCorlay]

Since the case of xtensor is already hard-coded into xeus-cling, xtl could be the first package to make use of the feature, ie. to install the JSON files in share/xcpp and etc/xcpp upon installation.

http://xeus-cling.readthedocs.io/en/latest/inline_help.html#enabling-the-quick-help-feature-for-third-party-libraries

This will probably require xhale

Cc @gouarin