Add module Features
Initial --load implementation

Loads plugins (.cmo, .cma or .cmxs) passed with the --load= option
to cilly.
Adapt plugin filename automatically
Build cilly with -linkall
Allow unsafe plugins
WIP
Move Plugin into Features, rewrite from scratch
Update ocaml-autoconf macro from upstream
Disable native build if natdynlink is not available
First implementation of findlib lookup
Add findlib to .merlin
Improve findlib interface
Fix natdynlink detection
Transform Callgraph and Epicenter into plugins