Support for extracting named parameters from multipart/form-data

[arnholm]

With reference to discussion in issue #310 and multipart/form-data it is requested that Crow adds functionality to extract named parameters in an easy and intuitive way exemplified by the multipart_params class (this code is just for reference, please feel free to use, modify or completely rewrite as needed).

[The-EDev]

One thing to add to this issue is that it would be useful to restructure the multipart::part struct to have a map of headers (as opposed to a vector) so they're accessible by string comparison rather than indices.

[dranikpg]

This fix sounds reasonable. I think there could be more, as I've always found the multipart api myself a bit hard/verbose to use.

One important missing feature is extracting parts by names. This has to be done manually now (by extracting the content-disposition header and searching for the name part 😵 )

Flask, for example, supports accessing data by name. Other web frameworks also do. I think we should provide at least some utility methods to make it working with multiple fields easier.