Unlike the previous ArgGroups, which made all args in the group mutually exclusive, one can now use
`ArgGroup::multiple(true)` which allows using more than one arg in the group (i.e. they're not all
mutually exclusive). When combined with `ArgGroup::required(true)` this effectively says, "At least
one of the args must be used, and using morethan one is also OK."
Closes#533
The `help` subcommand can now accept other subcommands as arguments to
display their help message. This is similar to how many other CLIs
already perform. For example:
```
$ myprog help mysubcmd
```
Would print the help message for `mysubcmd`. But even more, the `help`
subcommand accepts nested subcommands as well, i.e. a grandchild
subcommand such as
```
$ myprog help child grandchild
```
Would print the help message of `grandchild` where `grandchild` is a
subcommand of `child` and `child` is a subcommand of `myprog`.
Closes#416
ArgGroups can now be built from standalone YAML documents. This is
labeled as a fix because it should have been the case prior. A
standalone YAML document for a group would look something like
```
name: test
args:
- arg1
- arg2
required: true
conflicts:
- arg3
requires:
- arg4
```
This commit also keeps support building groups as part of a larger
entire App YAML document where the name is specified as a key to the
yaml tree
