Allow leading hyphens for arguments

[MartinKavik]

Fixes #534

This is one-line change, ignore the first two commits, I'll rebase it once #535 is merged.

This change allows to pass hyphenated arguments to tasks - e.g. makers my_task arg1 -arg2.

There are two ways to make it work:

1. Allow hyphenated args globally with AppSettings::AllowLeadingHyphen
2. Call Arg::allow_hyphen_values on Arg::with_name("TASK_ARGS")

The second option doesn't work (I don't know why) so I've chosen the first option in this PR.
Note: There are some warnings in the related doc comments. They look acceptable for me, but I recommend to look at them, maybe you'll find some surprising edge-cases that I don't see.

[sagiegurari]

@MartinKavik cool. changes in clap always make me feel a bit uneasy. i hope it won't break other things.
I don't have a good way to test it either :(
so will run few manual tests and then merge it.

thanks again for all the fixes. feel free to push PRs anytime :)

[sagiegurari]

@MartinKavik merging. thanks a lot for this.