Issue title: custom button send the form when onClick() be triggered
 Issue body: I create a Custom Input  to upload file to firebase,

code 

`   
     let { fieldApi } = this.props
     let getImage = e => {
		let file = e.target.files[0]
		let reader = new FileReader()
		console.log(reader.result)
		reader.onloadend = () => {
			console.log(reader.result)
			this.setState({ blob: reader.result })
		}
		reader.readAsDataURL(file)
     }
    let upload = () => {
			let { blob } = this.state
			let path = 'images/category'
			firebaseUpload(blob, path)
     }
    return (
	<div>
	   <input onChange={e => getImage(e)} type="file" />
	   <button onClick={e => upload}>addf</button>
	</div>
 )
`

when I click button to upload file the button also send the form 

Gif Demo

![https://media.giphy.com/media/l4pT0qV2legImpLG0/giphy.gif](https://media.giphy.com/media/l4pT0qV2legImpLG0/giphy.gif)
 Comments: 
 Comment 0: the default type of an html button is `submit` which will submit any form the button is inside. Try setting the `type="button"` on the button
 Comment 1: Yup be careful with type="button" as it will not submit a form.
{'source': 'GithubIssues', 'repo_name_with_owner': 'react-tools/react-form', 'issue_id': 292258492, 'repo_stars': 640.0}