Allow scenarios to be read from a file

[codeblooded]

Users of the CLI should be able to read the scenario JSON from a file; instead of, passing it directly as a flag. If we move to spf13/cobra (+ spf13/viper) for our CLI, I believe we will inherit reading configs with minimal effort.

Requested by: @paulosjca

[codeblooded]

We can also consider reading the scenario from a STDIN pipe. This may be useful.