Implement Quote Formatting

Adds support for block quote text formatting. This feature is currently only implemented in the Canary client.
This formatting adds a "> " to each new line from the input text.

src/Discord.Net.Core/Format.cs

@@ -1,3 +1,5 @@
+using System.Text;
+
 namespace Discord
 {
     /// <summary> A helper class for formatting characters. </summary>
@@ -37,5 +39,41 @@ namespace Discord
                 text = text.Replace(unsafeChar, $"\\{unsafeChar}");
             return text;
         }
+
+        /// <summary>
+        ///     Formats a string as a quote.
+        /// </summary>
+        /// <param name="text">The text to format.</param>
+        /// <returns>Gets the formatted quote text.</returns> // TODO: better xmldoc
+        public static string Quote(string text)
+        {
+            if (text == null)
+                return null;
+
+            StringBuilder result = new StringBuilder();
+
+            int startIndex = 0;
+            int newLineIndex;
+            do
+            {
+                newLineIndex = text.IndexOf('\n', startIndex);
+                if (newLineIndex == -1)
+                {
+                    // read the rest of the string
+                    var str = text.Substring(startIndex);
+                    result.Append($"> {str}");
+                }
+                else
+                {
+                    // read until the next newline
+                    var str = text.Substring(startIndex, newLineIndex - startIndex);
+                    result.Append($"> {str}\n");
+                }
+                startIndex = newLineIndex + 1;
+            }
+            while (newLineIndex != -1 && startIndex != text.Length);
+
+            return result.ToString();
+        }
     }
 }

test/Discord.Net.Tests.Unit/FormatTests.cs

@@ -28,5 +28,22 @@ namespace Discord
             Assert.Equal("```cs\ntest\n```", Format.Code("test", "cs"));
             Assert.Equal("```cs\nanother\none\n```", Format.Code("another\none", "cs"));
         }
+        [Fact]
+        public void QuoteNullString()
+        {
+            Assert.Null(Format.Quote(null));
+        }
+        [Theory]
+        [InlineData("", "> ")]
+        [InlineData("\n", "> \n")]
+        [InlineData("\n ", "> \n>  ")]
+        [InlineData("input", "> input")] // single line
+        // should work with CR or CRLF
+        [InlineData("inb4\ngreentext", "> inb4\n> greentext")]
+        [InlineData("inb4\r\ngreentext", "> inb4\r\n> greentext")]
+        public void Quote(string input, string expected)
+        {
+            Assert.Equal(expected, Format.Quote(input));
+        }
     }
 }