Add option --variables-file / --variables

[fabricereix]

Currently, variables can be set at the command-line with the option --variable <name=value>

It can become a bit tedious when the number of variables increases.
Hurl could also read variables from an input properties file with an option like --variables-file <FILE>

The values defined in the properties will be exactly the same as the one define din the command-line.

[fabricereix]

implemented in #118