Allow using collection accessors on strings (#245)

* allow using collection accessors on strings

* refactored resolving collection accessors

* refactored to fileprivate function

* Update Variable.swift

* Update templates.rst

Sources/Variable.swift

@@ -87,19 +87,9 @@ public struct Variable : Equatable, Resolvable {
           current = dictionary[bit]
         }
       } else if let array = current as? [Any] {
-        if let index = Int(bit) {
-          if index >= 0 && index < array.count {
-            current = array[index]
-          } else {
-            current = nil
-          }
-        } else if bit == "first" {
-          current = array.first
-        } else if bit == "last" {
-          current = array.last
-        } else if bit == "count" {
-          current = array.count
-        }
+        current = resolveCollection(array, bit: bit)
+      } else if let string = current as? String {
+        current = resolveCollection(string, bit: bit)
       } else if let object = current as? NSObject {  // NSKeyValueCoding
         #if os(Linux)
           return nil
@@ -128,6 +118,24 @@ public struct Variable : Equatable, Resolvable {
   }
 }
 
+private func resolveCollection<T: Collection>(_ collection: T, bit: String) -> Any? {
+  if let index = Int(bit) {
+    if index >= 0 && index < collection.count {
+      return collection[collection.index(collection.startIndex, offsetBy: index)]
+    } else {
+      return nil
+    }
+  } else if bit == "first" {
+    return collection.first
+  } else if bit == "last" {
+    return collection[collection.index(collection.endIndex, offsetBy: -1)]
+  } else if bit == "count" {
+    return collection.count
+  } else {
+    return nil
+  }
+}
+
 /// A structure used to represet range of two integer values expressed as `from...to`.
 /// Values should be numbers (they will be converted to integers).
 /// Rendering this variable produces array from range `from...to`.