Change contains_value to enum_contains (#31)

* change contains_value to enum_contains

* update test

* update doc

doc/reference.md

@@ -9,6 +9,7 @@
 * [`enum_names` obtains string enum name sequence.](#enum_names)
 * [`enum_entries` obtains pair (value enum, string enum name) sequence.](#enum_entries)
 * [`enum_index` obtains index in enum value sequence from enum value.](#enum_index)
+* [`enum_contains` checks whether enum contains enumerator with such value.](#enum_contains)
 * [`is_unscoped_enum` checks whether type is an Unscoped enumeration.](#is_unscoped_enum)
 * [`is_scoped_enum` checks whether type is an Scoped enumeration.](#is_scoped_enum)
 * [`underlying_type` improved UB-free "SFINAE-friendly" std::underlying_type.](#underlying_type)
@@ -43,7 +44,7 @@ constexpr optional<E> enum_cast(underlying_type_t<E> value) noexcept;
   * String to enum value.
 
     ```cpp
-    std::string color_name{"GREEN"};
+    string color_name{"GREEN"};
     auto color = magic_enum::enum_cast<Color>(color_name);
     if (color.has_value()) {
         // color.value() -> Color::GREEN
@@ -108,7 +109,7 @@ constexpr size_t enum_count() noexcept;
 * Examples
 
   ```cpp
-  constexpr std::size_t color_count = magic_enum::enum_count<Color>();
+  constexpr auto color_count = magic_enum::enum_count<Color>();
   // color_count -> 3
   ```
 
@@ -198,6 +199,48 @@ constexpr array<pair<E, string_view>, N> enum_entries() noexcept;
   // color_entries[0].second -> "RED"
   ```
 
+## `enum_index`
+
+```cpp
+template <typename E>
+constexpr optional<size_t> enum_index() noexcept;
+```
+
+* Obtains index in enum value sequence from enum value.
+
+* Returns `std::optional<std::size_t>` with index.
+
+* Examples
+
+  ```cpp
+  constexpr auto color_index = magic_enum::enum_index(Color::BLUE);
+  // color_index -> color_index.value() -> 1
+  // color_index -> color_index.has_value() -> true
+  ```
+
+## `enum_contains`
+
+```cpp
+template <typename E>
+constexpr bool enum_contains(E value) noexcept;
+constexpr bool enum_contains(underlying_type_t<E> value) noexcept;
+constexpr bool enum_contains(string_view value) noexcept;
+```
+
+* Checks whether enum contains enumerator with such value.
+
+* Returns true is enum contains value, otherwise false.
+
+* Examples
+
+  ```cpp
+  magic_enum::enum_contains(Color::GREEN); // -> true
+  magic_enum::enum_contains<Color>(2); // -> true
+  magic_enum::enum_contains<Color>(123); // -> false
+  magic_enum::enum_contains<Color>("GREEN"); // -> true
+  magic_enum::enum_contains<Color>("fda"); // -> false
+  ```
+
 ## `is_unscoped_enum`
 
 ```cpp