Add performance test (no reporting yet)

Tests/StencilTests/LexerSpec.swift

@@ -1,6 +1,7 @@
-import XCTest
+import PathKit
 import Spectre
 @testable import Stencil
+import XCTest
 
 class LexerTests: XCTestCase {
   func testLexer() {
@@ -115,4 +116,14 @@ class LexerTests: XCTestCase {
       }
     }
   }
+
+  func testPerformance() throws {
+    let path = Path(#file) + ".."  + "fixtures" + "huge.html"
+    let content: String = try path.read()
+
+    measure {
+      let lexer = Lexer(templateString: content)
+      _ = lexer.tokenize()
+    }
+  }
 }