Draft: QoL changes for packages and stuff

@kn This is a work in progress to improve some behaviours. The ones I want to fix right now are:

• use cmdliner instead of args (todo)
• use camlp-streams if OCaml version is > 4.14 (done)
• install the missing dependencies:
  • right now I did it with a configure file but I find it strange that pxp and js_of_ocaml are not set as mandatory dependencies since if they're not installed, make will fail

If you see something else, I'll be glad to do it in this MR before we merge it