Add commands option to bash completion

Add -c/--commands option to the bash completion script.

priv/shell-completion/bash/rebar

@@ -6,8 +6,8 @@ _rebar()
     COMPREPLY=()
     cur="${COMP_WORDS[COMP_CWORD]}"
     prev="${COMP_WORDS[COMP_CWORD-1]}"
-    sopts="-h -v -f -j"
-    lopts=" --help --verbose --force --jobs="
+    sopts="-h -c -v -f -j"
+    lopts=" --help --commands --verbose --force --jobs="
     cmdsnvars="analyze build_plt check_plt clean compile \
         create-app create-node eunit generate \
         install int_test perf_test test \