Move file reading responsibility to the CLI

[stevenocchipinti]

This means Scanner is a simple wrapper for LockfileParser and accepts
the same arguments (content in the form of a string, not a directory path).

This also means that Scanner is able to be called with content from
anywhere (for example, github's content API) instead of a hardcoded file
path and makes it more useful as a library other projects can depend on.

[postmodern]

Sounds reasonable. Perhaps we should add Scanner.parse and Scanner.open?

[stevenocchipinti]

Could do that. So just to clarify, would you imagine moving the file reading logic back in to Scanner and allow either of these invocations:

scanner = Scanner.open('Gemfile.lock')

and

scanner = Scanner.parse(File.read(File.join(root, 'Gemfile.lock')))

How would you expect #initialize to behave?