http language: external body file

[sassman]

Extension to the http language file format #5

A request body can be provided by referencing an external file.
The external file is provided by the < symbol in front of a valid absolute or relative file path. The whitespace between < and the file path is optional.

It is expected that the file is processed to replace all used variables as if they were specified inline.

Example

### Send POST request with json body
POST https://httpbin.org/post
Content-Type: application/json

< ./request-data.json