Export MobiledocParser and utils functions to support extensibility

[lukemelia]

Our use case for these additional exports is as follows:

MobiledocParser: we use it in a "cleaning" service that processes mobiledoc before saving to remove cards that have been added but not configured and so are effectively empty and should be removed.

parsePostFromHTML: we use it to let users convert older HTML content to mobiledoc, and in our tests to allow succinct creation of mobiledoc from HTML

parsePostFromText: we use it to convert text from a CSV import into mobiledoc for further editing in our UI

detect: we use it to find link markups in a mobiledoc to facilitate selection expansion when replacing a link

[lukemelia]

Looks like the build failure here is related to SauceLabs credentials.