fixed code quality issues using DeepSource

* Combine multiple `append`s into a single call

* Remove unnecessary use of slice

* Replace `strings.Index` with `strings.Contains`

* Added .deepsource.toml config file for DeepSource integration

.deepsource.toml

@@ -0,0 +1,17 @@
+version = 1
+
+test_patterns = ["**/*_test.go"]
+
+exclude_patterns = [
+    "_examples/**",
+    "_benchmarks/**",
+    ".github/**"
+]
+
+[[analyzers]]
+name = "go"
+enabled = true
+
+  [analyzers.meta]
+  import_paths = ["github.com/kataras/iris"]
+