Code cleanup + Use symlinks instead of resolver + Misc optimizations

[israelroldan]

This PR consists of a big refactor in order to improve the manageability of the codebase and make mondorepo as a cli tool more friendly to the end users.

Summary of the main changes:

• Use symlinks instead of a resolve hook: (Support symlinks to packages #24) By far this is the biggest change in this PR, and it aims to make mondorepo suitable to work with open source packages, removing the need to include mondorepo-specific code in projects.
• Use conf as configuration file backend: Allows for cross browser support and a clean API to get, set and clear configuration parameters.
• Remove ora as a dependency: For the time being, messages logged to the console are back to plain old lines. While ora is a nice-to-have, it was not correctly implemented and made the code difficult to maintain. We can revisit this in the future as an enhancement.
• Remove exec command: Since known packages are now symlinked, there's no need to have a dedicated exec command anymore. Plain old node script.js will do it just fine.
• Use npm instead of yarn: While a little buggy (see comments on cli/install.js, npm@5 offers some features that make yarn an unnecessary default, we can revisit adding it in the future.
• Split code into cli and lib: This will help us manage the codebase a bit better.