Add titleProp option

[gregberge]

As mentioned in #75, it would be nice to be able to specify a title using props. To do it, my proposition is to add a boolean option "titleProp" defaulting to false. When activated, user would be able to specify the title of the SVG using props.

Usage in command line

svgr --title-prop

Usage in Node / Webpack

transform("...", { titleProp: true })

Usage in React, when the option is activated

import SvgrComponent from './SvgrComponent'

<SvgrComponent title="This is a title" />
// The output will be:
// <svg><title>This is a title</title></svg>
// Instead of:
// <svg title="This is a title" />