Hide flag completion unless input ends with a hyphen

[DannyBen]

This change makes it so the returned completions will not include any flags (--flags) unless the input line ends with a -.

In other words, given this config:

# completely.yaml
mygit:
- --help
- --version
- status
- init
- commit

The new behavior will be:

$ completely test "mygit "
status
init
commit

$ completely test "mygit -"
--help
--version

instead of:

$ completely test "mygit "
status
init
commit
--help
--version

cc #32

[DannyBen]

Pattern now receives a function name, which is the name of the helper filter function. It is used in Pattern#compgen to generate the compgen string which now calls a function rather than just using the string straight up.

[DannyBen]

The function at line 7 receives a space-delimited string of possible completions, and:

1. returns the same string if the current word starts with a hyphen
2. returns a new string without any --flags if the current word does not start with a hyphen

[DannyBen]

Since completions are now returned differently if the input ends with a hyphen, this integration spec config was expanded to list both cases.

[mrliptontea]

I think this works, nice.

[DannyBen]

Merged, will release later.
If you are using the ruby version (and not the docker) you can already test it if you wish.