Fix conditionals to use [[ and (( rather than [

This fixes a small issue in src/widgets.zsh which makes it so if you
alias [ to g[ (as is done in prezto if the gnu-utility module is loaded)
autosuggestions would fail.

The documentation for GNU test mentions that -o and -a should be avoided
if possible because it's not very clear. Also, with zsh and [[ -o
actually tests if an option is set, which makes this option even more
confusing.

spec/integrations/rebound_bracket_spec.rb

@@ -0,0 +1,13 @@
+describe 'rebinding [' do
+  context 'initialized before sourcing the plugin' do
+    before do
+      session.run_command("function [ { $commands[\\[] \"$@\" }")
+      session.clear_screen
+    end
+
+    it 'executes the custom behavior and the built-in behavior' do
+      session.send_string('asdf')
+      wait_for { session.content }.to eq('asdf')
+    end
+  end
+end