Ignore Ctrl-C in `honcho run`

When running ad hoc commands under Honcho with `honcho run`, Honcho currently does not catch `KeyboardInterrupt` or give the spawned process an opportunity to handle Ctrl-C. In addition to an ugly `KeyboardInterrupt` traceback, this can cause the terminal to be left in an inconsistent state. 

For instance, when running a command that switches the terminal to raw mode (such as one that uses the readline library), the terminal is left in raw mode after honcho exits, requiring an `stty sane` to restore it. This occurs frequently in my environment, where we use `honcho run python` quite often.

This patch causes the `honcho run` command to ignore Ctrl-C (SIGINT) altogether, allowing the spawned command process to handle it however it sees fit.