Fixes #61.

The completion is rather simplistic:

• Just suggest all possible flags, all the time.
• If the current word doesn't start with a hyphen, also suggest all files and dirs.

That makes it quite usable, because "flags and files" already covers most usages.

Fancy stuff that is NOT implemented:

• All ValueHint logic (in particular Strings(Vec<String>), DirPath, ExecutablePath, Username, Hostname would be reasonably easy)
• dd-style arguments
• Mutually-exclusive flags/arguments (i.e. if -a and -b can never go together and -a was already given, then don't suggest -b)

All of these features would necessarily need to be implemented in bash, which doesn't sound fun.