Allow disabling of exec interpolation

Currently when using the nodemon API, the string passed as "exec" is split into multiple arguments if it contains a space. This makes it impossible to specify an executable with a space in its path.

This PR allows programmatic API usage to specify `execShell:false` to disable this behaviour.

The second commit exposes this flag via the cli, as `--no-exec-shell`. If you don't want this, feel free to leave it off (I only use the API personally, so I only exposed it on the CLI for completeness).

I've the existing behaviour enabled by default to prevent functionality changes, although I personally think that interpolation should be opt in. Or even better, require multiple `-x` options in order to build up a proper array (like `-x python -x -v`), rather than introducing additional syntax. But both of those options require more work, and are backwards incompatible.