Support for custom pty options

Especially when we need have to use custom environment variables.

Example:

./bin/web-term --pty-options '{ "env": { "MY_ENV_VARIABLE": "..." } }'

When using it as library:

let myTerm = new WebTerm({
   cols: 80
 , rows: 60
 , socket: ...
 , ptyOptions: {
      env: {
          WEB_TERM_USER: "my-web-term-user",
          WEB_TERM_HOST: "my-web-term-host",
       }
   }
});

IonicaBizau / web-term

Support for custom pty options #47