<Button /> icon improvements

[idmontie]

Improvement

Make it easier to use MDI icons with the <Button /> component. Add an iconOnly prop.

Example Usage

Currently, you need to do the following if you want an icon only button:

<Button text onClick={open}>
    <Icon
	path={mdiCheck}
	title="Added"
	color="currentColor"
	size={1}
	style={{ verticalAlign: 'middle' }}
    />
</Button>			

It should be usable through the icon prop:

<Button
    text
    icon={(
      <Icon
	  path={mdiCheck}
	  title="Added"
	  color="currentColor"
      />
    )}
    iconOnly    
</Button>