Implement substitute method on TextRun class

Enables substituting text in multi-line paragraphs, preserving formatting and text styles.

Example usage:
```
doc.paragraphs.each do |p|
  p.each_text_run do |tr|
    tr.substitute('_placeholder_', 'replacement value')
  end
end
```

Author: misdoro

README.md

@@ -89,6 +89,13 @@ doc.paragraphs.each do |p|
   p.remove! if p.to_s =~ /TODO/
 end
 
+# Substitute text, preserving formatting
+doc.paragraphs.each do |p|
+  p.each_text_run do |tr|
+    tr.substitute('_placeholder_', 'replacement value')
+  end
+end
+
 # Save document to specified path
 doc.save('example-edited.docx')
 ```

lib/docx/containers/text_run.rb

@@ -45,6 +45,13 @@ def parse_text
           @text_nodes.map(&:content).join('')
         end
 
+        # Substitute text in text @text_nodes
+        def substitute(match, replacement)
+          @text_nodes.each do |text_node|
+            text_node.content = text_node.content.gsub(match, replacement)
+          end
+        end
+
         def parse_formatting
           {
             italic:    !@node.xpath('.//w:i').empty?,

spec/docx/document_spec.rb

@@ -166,6 +166,24 @@
     end
   end
 
+  describe 'format-preserving substitution' do
+    before do
+      @doc = Docx::Document.open(@fixtures_path + '/substitution.docx')
+    end
+
+    it 'should replace placeholder in any line of a paragraph' do
+      expect(@doc.paragraphs[0].text).to eq('Page title')
+      expect(@doc.paragraphs[1].text).to eq('Multi-line paragraph line 1_placeholder2_ line 2_placeholder3_ line3 ')
+
+      @doc.paragraphs[1].each_text_run do |text_run|
+        text_run.substitute('_placeholder2_', 'same paragraph')
+        text_run.substitute('_placeholder3_', 'yet the same paragraph')
+      end
+
+      expect(@doc.paragraphs[1].text).to eq('Multi-line paragraph line 1same paragraph line 2yet the same paragraph line3 ')
+    end
+  end
+
   describe 'read formatting' do
     before do
       @doc = Docx::Document.open(@fixtures_path + '/formatting.docx')