support --value/--argument param for use with --execute

at the moment, there is no way to safely interpolate values into SQL that is passed into crash on the command-line for execution. the only way to do it is via shell string interpolation

it would be good if crash supported something like `--variable=assignment` ala the psql client (cf. https://www.postgresql.org/docs/9.5/app-psql.html)

at the moment, I have to warn users not to attempt something like this in production, even though it's handy for use in tutorials. see the WARNING note here: https://crate.io/docs/crate/tutorials/en/latest/generate-time-series/cli.html#record-the-iss-position