Implement the `goto_scene` command.

The goto_scene command has been implemented; with that, it will be possible to use more than one source script on the project.

Variables and conditionals.

• Commands create, temp and set implemented, in order to allow string creation/manipulation
• Implemented expression support;
• Commands if, elseif and else implemented, in order to allow conditional execution

Implement `sound` and `stop` commands.

Add the `stop` command to the README.

Merge pull request #20 from haroldo-ok/implement-sound

First prototype

More commands

Merge pull request #5 from haroldo-ok/more-commands