Add OCaml package manager (OPAM) (#62)

This adds support for the OCaml package manager (OPAM).

The command line `opam update` first performs a self-update if
necessary. Then it updates its package list.
Finally, `opam upgrade` upgrades all locally installed packages.

src/main.rs

@@ -204,6 +204,10 @@ fn run() -> Result<(), Error> {
         |terminal| generic::run_emacs(&base_dirs, terminal, dry_run),
         &mut terminal,
     ));
+    report.push_result(execute(
+        |terminal| generic::run_opam_update(terminal, dry_run),
+        &mut terminal,
+    ));
     report.push_result(execute(
         |terminal| vim::upgrade_vim(&base_dirs, terminal, dry_run),
         &mut terminal,