Add <Input> layout block and block element

[yhatt]

This PR has implemented a part of #57.

jsx-slack will provide <Input> component for 2 usages. A one is for layout block as same as an original spec in Slack API, and the other is focusing to the compatibility with HTML as like as existing <Input> component for Dialog support.

<Input> layout block

<Input> layout block should contain one of available elements such as <Select>.

<Modal title="My App">
  <Input label="User" hint="Please select one of users." required>
    <UsersSelect placeholder="Choose user..." />
  </Input>
</Modal>

<Input> block element

For compatibility of HTML, we adopted to use <Input> component as an element instead of providing <PlainTextInput> element component.

<Modal title="My App">
  <Input label="Title" name="title" maxLength={80} required />
</Modal>

At the moment, multiline prop is not implemented in <Input>. It would be provided in another component like <Textarea>.