Multiple request encodings

[danirod]

Currently only raw payloads are supported, but the following encodings should be supported:

• application/x-www-form-urlencoded (the standard for forms, will encode key-value parameters).
• multipart/form-data (mostly useful for attaching files, so it is also a dependency)
• RAW (current mode, although maybe having a dropdown to pick the content type and to colorize the input field could be good).