Improve multipart content reader interface

This is another breaking change.

Digest Authentication support

v0.3.3

Updated README

Removed C++ exception

v0.3.2

Fix #282

Improve multipart content reader interface

Improve multipart content reader interface. It's another breaking change...

The MultipartFile structure has been modified.

The MultipartFile structure has been modified. This is a breaking change.

https://github.com/yhirose/cpp-httplib/blob/d910bfc303abb8c337f18223fb92b04044ce087f/httplib.h#L210-L214

v0.2.6

Added upload example

v0.2.5

Fixed PRI request problem

v0.2.4

Update README

v0.2.3

Fixed build error on Windows

v0.2.2

Fixed problem caused by former changes