Add an argument to function `run_command`

This [function ](https://github.com/KarlHeitmann/rg_wrapper/blob/8178db2cba9cfbb9141bb06887918004f859b40a/src/io.rs#L4) has the argument hard coded in it's body. Extract the argument hard coded in the call to `rg` and put it as an input argument on the function.

Open src/io.rs and inspect the run_command function and its call to rg. Extract the hard-coded argument from that call into the function's input arguments. Done means run_command accepts the argument and passes it to rg instead of keeping it hard-coded.

Written by the indexing model from the issue text.