Add support for PreparsedDocumentProvider

[jord1e]

See #233 (do you really need to make an issue in situations like these?)

[bclozel]

Hello @jord1e

Submitting just a PR is fine as long as it's not too large and explains the changes.
We'd rather have issues as starting point when PRs represent an important investment of contributor's time and we're likely to decline it as an outcome of the discussion.

In this case this looks good so I'll close this issue.

Closing in favor of #233