add snippets about ES2015 module syntax (import, export) (#219)

* add snippets about ES2015 module syntax (import, export)

For details, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import

* follow exact 'import' syntax

* add 'group: es6'

snippets/js-mode/export

@@ -0,0 +1,6 @@
+# -*- mode: snippet; require-final-newline: nil -*-
+# name: export
+# key: exp
+# group: es6
+# --
+export${1: default} $0;

snippets/js-mode/import

@@ -0,0 +1,6 @@
+# -*- mode: snippet; require-final-newline: nil -*-
+# name: import
+# key: imp
+# group: es6
+# --
+import ${1:defaultMember}${2:, { ${3:member} }} from '${4:module}';

snippets/js-mode/import-member

@@ -0,0 +1,6 @@
+# -*- mode: snippet; require-final-newline: nil -*-
+# name: import member
+# key: imm
+# group: es6
+# --
+import { ${1:member1}, ${2:member2} } from '${3:module}';