merlin: update from 1.7.1 to 2.0

Also adds an option to compile the emacs mode at install time.
Add henrytill to maintainers
opam: update to 1.2.0
Merge pull request #4808 from henrytill/opam

opam: update to 1.2.0
Merge pull request #4810 from vbgl/merlin

merlin: update from 1.7.1 to 2.0
audit: update from 2.4 to 2.4.1