Allow injecting Vue

[fjc0k]

This PR is helpful for component developers to show their components, e.g.

.vuepress/inject.js:

import FirUI from '../../dist/fir-ui.min.js'
import '../../dist/fir-ui.min.css'

export default Vue => {
  Vue.use(FirUI)
}

components/button.md:

<f-button type="primary">Button</f-button>

[ulivz]

Since the new version has supported to eject the default theme (89538fa) which can let you to mutate the Vue constructor, so I'm not sure whether it's necessary. Maybe this PR is good for those users who don't want to eject theme but still want to extend Vue.

[fjc0k]

@ulivz The new feat EJECT is good to develop a new theme. But most of the time, we just need to extend Vue. What's more, we want keep it independent on THE THEME.

[egoist]

maybe adding a webpack alias to this file and import it in lib/app/app.js would be better

[ulivz]

IMO, This solution would be better.

[egoist]

And there's no need to inject Vue as the argument since you can directly import it, something else like app instance may be be more useful.

[fjc0k]

#80