Add "skip-prepare" Option to Install

[morinap]

• This change adds a --skip-prepare option to the install mix task and passes an opts Keyword list to the install function for handling. If the skip option is true, the prepare_app_css and prepare_app_js functions will not be called.
• It also moves the creation of assets/css into prepare_app_css so that the folder is not created unless necessary
• It also updates the mix task to display some detail on the options in its usage statement, as well as to properly display the usage statement if an invalid usage is presented.

This fixes https://github.com/phoenixframework/tailwind/issues/44

[josevalim]

Oops, I have already pushed it as --no-assets. If anything is pending, please feel free to submit another PR (and sorry!) :)