Add File Plugin

Added the File Plugin based off of https://gist.github.com/maleeb/3bed0cf3d51831909a61, and added `uploadFile` and `downloadFile` methods.

It's very buggy at the moment, and needs testing, but it's a starting point.

Here are the TODOS:
 - writeFile needs work, doesn't function
 - add support for readFile -> readAsData
 - add support for readFile -> readAsBinaryString
 - add support for readFile -> readAsArrayBuffer
 - add functionality to define storage size in the getFilesystem() -> requestFileSystem() method
 - add documentation for FileError types
 - add abort() option to downloadFile and uploadFile methods.
 - add support for downloadFile and uploadFile options. (or detailed documentation) -> for fileKey, fileName, mimeType, headers
 - add support for onprogress property in upload and download