Remove shared directory by default or w/ flag

[djfarrelly]

Purpose

Inspired by the discussion in this PR, we can remove the shared directory prefix to allow for cleaner manifests and easier usage. This would make the @bufferapp/static-assets library simpler and custom solutions to be cleaner as well.

Idea

Given a generated manifest:

{
  "public/js/bundle.js": "https://static.buffer.com/my-app/public/js/bundle.123456789.js"
}

It would become:

{
  "js/bundle.js": "https://static.buffer.com/my-app/js/bundle.123456789.js"
}

Since most apps are never referencing ./public in their code, it makes sense to remove it, potentially by default. To do this we could:

• Automatically remove the word public as a prefix directory
• Check all file globs and remove any common base directory that is shared by all
• Change the structure of the globs to be simpler or allow for a directory prefix input, ex. `buffer-static-upload -prefix public -files 'js/.js,css/.css'

[philippemiguet]

@djfarrelly, one potential issue with this is, if someone uploads 2 files with the same name, one from the root dire ./ and the other from ./public, it will result in conflicting keys. That's a edge case those and shouldn't happen.

Usually the files in the root dir are gulp generated files. We could share a good practice to generate those files in a specific directory ./build for instance, and never upload files from the root directory :)

[djfarrelly]

Good call @philippemiguet, that will likely be another check we should add in a function like EnsureUniqueKeys that throws and error if there are duplicates.