The previous format of documentation made it look like `--message VALUE`
(no equal sign) would be supported, which might be misleading.

There is no need to detect the current branch if a range is provided
as an argument.

Fixes #1584.

Fixes #1758

The old `args.After("echo", ...)` approach won't cut it on Windows since
there is no `echo` executable. Thus, nothing was printed on Windows.

Instead, expand Args type with extra functionality that allows Go
callbacks after all commands in the chain have been run.

Also, the new `args.NoForward()` is now preferred to `os.Exit()` since
it's more descriptive and allows `defer` in the calling function.

It's especially important to encode the `#` character, because it would
get interpreted as a URI fragment identifier. However, we let
`url.QueryEscape` encode any other unsafe characters, except allowing
special characters like `/!:^~` to appear raw. Most of these characters
aren't allowed in branch names anyway, but they can appear as git
revision expressions such as `feature^` or `mislav:feature~3`.

Pull in Hub compatible fixes in https://github.com/github/hub/commit/f60fb89274d276034e5c7c2d65f27d36dcfac685

So we can use flags before and after arguments, and we can group boolean flags, just like git does.

- Use a hash to lookup for commands.
- Add more tests for the command and runner execution.

Since https://github.com/github/hub/compare/31a85e519e0a080dd10ce21339f806a9a0877766...master

Some tests are ignored either because they don't make sense or WIP