Use hooks to generate rebar3 executable faster

Repeats some of the steps used in the bootstrap script for non-windows
systems so that 'rebar3 escriptize' can be used in development for
faster results:

- ./bootstrap  9.95s user 1.55s system 102% cpu 11.234 total
- rebar3 escriptize  0.49s user 0.14s system 101% cpu 0.623 total

rebar.config

@@ -12,6 +12,10 @@
         {cth_readable,        "1.2.3"},
         {eunit_formatters,    "0.3.1"}]}.
 
+{post_hooks, [{"(linux|darwin|solaris|freebsd|netbsd|openbsd)",
+               escriptize,
+               "cp $REBAR_BUILD_DIR/bin/rebar3 ./rebar3 && chmod u+x rebar3"}]}.
+
 {escript_name, rebar3}.
 {escript_emu_args, "%%! +sbtu +A0\n"}.
 %% escript_incl_extra is for internal rebar-private use only.