Adds new method/attribute `Arg::value_hint`, taking a `ValueHint` enum
as argument. The hint can denote accepted values, for example: paths,
usernames, hostnames, commands, etc.
This initial implementation supports hints for the zsh and fish
completion generators, support for other shells can be added later.
compgen takes exactly one "word" argument; if multiple are passed, it
will ignore all but the first. If `$cur` is not quoted and contains
whitespace, it will be expanded into multiple words, so the completions
will be based only on the first word of `$cur`, which is clearly not
correct.
To fix this, quote `$cur` to prevent word splitting, where necessary
(Bash does not perform word splitting inside the extended test command
`[[`, so quotes aren't necessary there).
