add cli execution and examples

[daniel-garcia]

This PR adds file watching to the cli interface. It also adds the sprig library for command and argument interpolation. The default config file also shows how to use the shelldb driver for executing shell commands in response to input changes.

./dsnexec run
INFO[0000] map[name:json source raw_dsn:{
     name: json source
}]  shelldb_destination="my shell program"