Replace variables in file paths

[skuro]

This change allows to specify a filename as a variable, e.g.:

:test-file = ./test-data.json

# Process stuff
GET http://localhost:8000/process
Content-Type: application/json

< :test-file

This allows to easily switch between different payloads for the same API with minimal efforts.

[leetwinski]

double this! We should merge this one somehow.