Complete v2

This is a major change in the package.
I saw that major clients are using go modules, so for them this transition should be transparent.
This change provides:

• Improved completion logic, from completion order to treating quotes and spaces.
• Expose the Completer interface, which enables any CLI library to use this completion implementation.
• Simplify and improve library API.
• Add automatic flag bash completion for the standard library flag package.
• Add ./compflag pacakge, which provides standard library flags with bash completion enables.