Add support of Scalar aliases

Author: IvanGoncharov

spec/Section 3 -- Type System.md

@@ -233,6 +233,30 @@ as `4`) input value should be coerced to ID as appropriate for the ID formats
 a given GraphQL server expects. Any other input value, including float input
 values (such as `4.0`), must raise a query error indicating an incorrect type.
 
+### Scalar aliases
+
+Scalar aliases should be used to add specific semantic or validation on top
+of the existing scalar type.
+
+Note: only scalar types can be aliassed, not other aliases.
+
+For example, a type `DateTime` could be described as:
+
+```
+scalar alias DateTime = String
+```
+
+Where `DateTime` is the name of alias and `String` is the name of the base scalar type.
+
+**Result Coercion**
+
+Coertion of the scalar alias value should include coertion of base type as the
+last step.
+
+**Input Coercion**
+
+Coertion of the scalar alias value should include coertion of base type as the
+first step.
 
 ### Objects
 

spec/Section 4 -- Introspection.md

@@ -146,7 +146,7 @@ type __Type {
   # INPUT_OBJECT only
   inputFields: [__InputValue!]
 
-  # NON_NULL and LIST only
+  # SCALAR, NON_NULL and LIST only
   ofType: __Type
 }
 
@@ -231,7 +231,8 @@ actually valid. These kinds are listed in the `__TypeKind` enumeration.
 
 #### Scalar
 
-Represents scalar types such as Int, String, and Boolean. Scalars cannot have fields.
+Represents scalar types such as Int, String, and Boolean or scalar aliases.
+Scalars cannot have fields.
 
 A GraphQL type designer should describe the data format and scalar coercion
 rules in the description field of any scalar.
@@ -241,6 +242,7 @@ Fields
 * `kind` must return `__TypeKind.SCALAR`.
 * `name` must return a String.
 * `description` may return a String or {null}.
+* `ofType`: must return a base scalar type if alias or {null}.
 * All other fields must return {null}.