Add yarn as alternative --with-yarn

[fooflare]

As I mentioned in #187 I made some changes to have yarn as option with --with-yarn flag. It works following the specifications below:

1. With new:
   1.1. If flag is specified:

   • Checks if yarn is installed and if not, installs it and changes the default command from npm to yarn (adding equivalent arguments)
   • Inits the yarn project, downloads and installs webpack dependencies

   1.2. If not, npm is executed

2. With generate webpack, I added the same flag as alternative:
   2.1. Checks if yarn.lock already exists and if so, yarn is established as default
   2.2. If yarn.lock doesn't exist:

   • If flag was specified, yarn is selected (same process than with new command)
   • If not, npm is executed

[markbates]

This looks good, but to be feature complete it needs to support the following commands:

• buffalo new
• buffalo dev
• buffalo build

Those commands are all touch the "asset pipeline".

I also think we should rename --skip-webpack to --skip-assets.

[fooflare]

One thing I haven't clear is about dev and build. I checked both but is webpack directly executed, not npm. Or am I wrong?

In reference to the renaming, I am not sure if --skip-assets or maybe --skip-npm. In this PR if webpack is skipped, yarn is skipped too, even if the flag is specified (no dependencies to be generated). And assets are generated in public in any case (application.js/.css and logo), so I am not sure if it would be totally clear.

[markbates]

Ok, I tested this and validated that all three commands work with a yarn application. Fantastic. Thanks for this.

[fooflare]

Don't thank me, I did very few. Thanks to you for all your efforts in the project.

[markbates]

It's a group effort! :)

[philipithomas]

👍