Add spec for chainable `parse` and `applyPlugins`

Issue: https://github.com/wooorm/retext/issues/20
Add chainable `parse` and `applyPlugins`

Issue: https://github.com/wooorm/retext/issues/20
Merge branch 'feature/chainable-parse'

Add chainable `parse` and `applyPlugins`

Due to changes in 27dbfd7, the `parse` and `applyPlugins`
methods no longer return a node. This leaves the option open
to, instead of returning void, be more chainable by returning
`self`.

Closes: https://github.com/wooorm/retext/issues/20