Add support for get-in/2

[oubiwann]

A la Clojure's get-in function.

Support the following types by atom keys

• proplists
• dicts
• orddicts

Support the following types by integer indices:

• nested lists (using nth)
• proplists (using element/2)

This blocks the following:

• https://github.com/lfex/lcfg/issues/3
• https://github.com/lfex/lcfg/issues/2

[oubiwann]

Lovely: for nested lists, it's just this simple:

(lists:foldl #'lists:nth/2 data indices))

[oubiwann]

And for nested proplists, just as beautiful:

(lists:foldl #'proplists:get_value/2 data keys)

[oubiwann]

Released as part of 0.6.1.