Add rebar plugin

[rebar](https://github.com/rebar/rebar)  is an Erlang CMD tool.

Add `rebar` to `.zshrc`:

```
plugins=(git rebar)
```

Then you can use the autocompletion:

```
> rebar
appid             -- Application id
apps              -- Application names to process
case              -- Common Test case
check-deps        -- Display to be fetched dependencies
clean             -- Clean
compile           -- Compile sources
create            -- Create skel based on template and vars
create-app        -- Create simple app skel
create-node       -- Create simple node skel
ct                -- Run common_test suites
delete-deps       -- Delete fetched dependencies
doc               -- Generate Erlang program documentation
dump_spec         -- Dump reltool spec
eunit             -- Run eunit tests
generate          -- Build release with reltool
generate-appups   -- Generate appup files
generate-upgrade  -- Build an upgrade package

omitted ...
```