Add CLI support

[lukehorvat]

Hey, cool module. 🙂 It would be nice if you could run it via the CLI.

This PR adds some basic CLI support, so you can run bash-parser like so:

bash-parser 'echo ciao' # if the package is installed globally

npx bash-parser 'echo ciao' # if the package is installed locally 

Outputs the AST as JSON.

[thetayloredman]

I'd suggest adding file support with --file <path>!