Update meow from v5 to v6

[kinueng]

Fixes #15

• meow v6.1.1 updated yargs-parser to v18.1.2, which contains a fix for
  a Prototype Pollution vulnerability reported in npm audit.

Testing

npm run-script test output

> [email protected] test /Users/kueng/work/sandboxes/del-cli
> xo && ava



  1 test passed

npm audit --production output

Using npm v6.14.5

                       === npm audit security report ===                        
                                                                                
found 0 vulnerabilities
 in 147 scanned packages

[kinueng]

Updated description with testing with npm audit after upgrading meow to v6.1.1

[sindresorhus]

To be clear, there's no vulnerability here as meow is not vulnerable as it doesn't support nested flags, but I'm ok with updating to remove the audit.