Add ContainMultiline and tests

Functions/Assertions/ContainMultiline.Tests.ps1

@@ -0,0 +1,20 @@
+Set-StrictMode -Version Latest
+
+InModuleScope Pester {
+    Describe "PesterContainMultiline" {
+        Context "when testing file contents" {
+            Setup -File "test.txt" "this is line 1`nthis is line 2`nPester is awesome"
+            It "returns true if the file matches the specified content on one line" {
+                "$TestDrive\test.txt" | Should ContainMultiline  "Pester"
+            }
+
+            It "returns true if the file matches the specified content across multiple lines" {
+                "$TestDrive\test.txt" | Should ContainMultiline  "line 2`nPester"
+            }
+
+            It "returns false if the file does not contain the specified content" {
+                "$TestDrive\test.txt" | Should Not ContainMultiline  "Pastor"
+            }
+        }
+    }
+}