einhornsh: Allow command execution outside of interactive shell

#### Commit summary

This commit does two things:

1. Allows commands to be executed outside of the Einhorn shell with `-e` or `--execute` to increment the number of workers.  
Example: `einhornsh -e inc` will spin up the number of workers by one.
2. Allows the use of semicolons as command delimiters both in and out of the Einhorn shell.  
Example: To spin up the number of workers by three, either `einhornsh -e "inc;inc;inc;"` outside of the shell or `inc;inc;inc;` inside the shell will work.

#### Edge cases
1. After splitting commands by semicolon, all empty commands are ignored.  
Example: `inc;;;`, `  ; inc; ;  `, and `inc` all increment the number of workers by one.
2. Any `quit` or `exit` before the end of a semicolon-delimited multi-command line returns from the shell, preventing later commands from running.  
Example: `inc; exit; inc;`, `inc; quit; inc; inc;`, and `inc` all increment the number of workers by one.