String::Incremental: `$foo = 'abc'` is available as synonym of `$foo->set('abc')`, using `tie`. refs #2
String::Incremental: update document. refs #2
oops, forgot updating README. refs #2
Merge pull request #3 from issm/id/2

using 'tie', `$foo = "abc"` is available as synonym of `$foo->set("abc")`