Add HTML <em> tag to core components

Closes #188

Author: citizen428

app/concepts/matestack/ui/core/em/em.haml

@@ -0,0 +1,5 @@
+%em{@tag_attributes}
+  - if options[:text].nil? && block_given?
+    = yield
+  - else
+    = options[:text]

app/concepts/matestack/ui/core/em/em.rb

@@ -0,0 +1,5 @@
+module Matestack::Ui::Core::Em
+  class Em < Matestack::Ui::Core::Component::Static
+
+  end
+end

spec/usage/components/em_spec.rb

@@ -0,0 +1,60 @@
+# frozen_string_literal: true
+
+require_relative '../../support/utils'
+include Utils
+
+describe 'Em Component', type: :feature, js: true do
+  it 'Example 1 - yield, no options[:text]' do
+    class ExamplePage < Matestack::Ui::Page
+      def response
+        components do
+          # simple em tag
+          em do
+            plain 'Emphasized text'
+          end
+
+          # enhanced em tag
+          em id: 'my-id', class: 'my-class' do
+            plain 'Enhanced emphasized text'
+          end
+        end
+      end
+    end
+
+    visit '/example'
+
+    static_output = page.html
+
+    expected_static_output = <<~HTML
+      <em>Emphasized text</em>
+      <em id="my-id" class="my-class">Enhanced emphasized text</em>
+    HTML
+
+    expect(stripped(static_output)).to include(stripped(expected_static_output))
+  end
+
+  it 'Example 2 - render options[:text]' do
+    class ExamplePage < Matestack::Ui::Page
+      def response
+        components do
+          # simple em
+          em text: 'Emphasized text'
+
+          # enhanced em
+          em id: 'my-id', class: 'my-class', text: 'Enhanced emphasized text'
+        end
+      end
+    end
+
+    visit '/example'
+
+    static_output = page.html
+
+    expected_static_output = <<~HTML
+      <em>Emphasized text</em>
+      <em id="my-id" class="my-class">Enhanced emphasized text</em>
+    HTML
+
+    expect(stripped(static_output)).to include(stripped(expected_static_output))
+  end
+end