Allow exposing stdin to scripts

[kpcyrd]

In some cases we want to pass a file to a script. Since we don't expose any functions to access the filesystem this is currently not possible, but we could add an --stdin mode that enables some functions to access stdin. This would allow parsing some text based files or output of some tools:

cat dhcpd.leases | sn0int run --stdin -f ./dhcpd-parser.lua