Bug Fix & Leveraging of Gatsby's Cache API

This release address an issue causing custom formats to be ignored, and also leverages Gatsby's cache API to avoid fetching Dropbox documents that haven't changed, resulting in faster builds.

Thanks to @g3ortega and @jamessimone for these respective contributions.

Support Document Pagination

• Adds pagination handling, so that applications can pull more than 1000 documents from the API.
• Clean up/improve code handling, including pulling functionality out of base class for less exposure of methods.

Support Automatic Transformation of Markdown

Sets mediaType to text/markdown, enabling gatsby-transformer-remark to transform pulled data.