Add enable/disable/toggle widgets to disable suggestion functionality

[GitHub #219]

Intended to be helpful for folks using bracketed-paste-magic and other
widgets that use `zle -U`.

spec/widgets/enable_spec.rb

@@ -0,0 +1,21 @@
+describe 'the `autosuggest-enable` widget' do
+  before do
+    session.
+      run_command('typeset -g _ZSH_AUTOSUGGEST_DISABLED').
+      run_command('bindkey ^B autosuggest-enable')
+  end
+
+  it 'enables suggestions and fetches a suggestion' do
+    with_history('echo world', 'echo hello') do
+      session.send_string('echo')
+      sleep 1
+      expect(session.content).to eq('echo')
+
+      session.send_keys('C-b')
+      wait_for { session.content }.to eq('echo hello')
+
+      session.send_string(' w')
+      wait_for { session.content }.to eq('echo world')
+    end
+  end
+end