[add] Clipboard API

Close #125
Fix #122

docs/apis/Clipboard.md

@@ -0,0 +1,16 @@
+# Clipboard
+
+Clipboard gives you an interface for setting to the clipboard. (Getting
+clipboard content is not supported on web.)
+
+## Methods
+
+static **getString**()
+
+Returns a `Promise` of an empty string.
+
+static **setString**(content: string): boolean
+
+Copies a string to the clipboard. On web, some browsers may not support copying
+to the clipboard, therefore, this function returns a boolean to indicate if the
+copy was successful.