Runcommands

- Support Python 3.8.
- Don't attempt to parse command args after `--` as multi short options. Args
after `--` should be passed verbatim.
- When determining if help was requested for a command, ignore args after `--`.
Args after `--` aren't command options.

- Allow env-specific default args, which take precedence over top level default
args (the default default args).

- Default args read from config are now checked and a useful exception is
raised if an unknown arg is encountered.
- Fixed/improved handling of container args. This deconflates the type of
a container from the type of values it contains. It also allows commands to
use `*args` to collect an arbitrary number of positional args.
- Fixed handling of `--` on the command line. Now, each command can make use of
`--` to specify the end of options and that the remaining args are
positional.
- The command to run via the `local` or `remote` commands no longer needs to be
quoted on the command line.
- Improved parsing and expansion of grouped short options like `-xyz`.

- Upgraded PyYAML 3.13 => 5.1. Depending on your setup, this may require
upgrading PyYAML to 5.1 first before upgrading to this version of
RunCommands (e.g., if you have a command that wraps `pip install`).

- Arg values are now collected into tuples when appropriate--when an arg's
default value is a tuple or its type is explicitly set to `tuple`.
- Positional args can now be specified with a type of `dict`, `list`, or
`tuple`. Previously, specifying one of these types didn't work as expected
because only one value would be read from the command line and that value
would be converted to a `list` or `tuple` of characters (and using `dict`
would cause an error).
- Multiple short options can now be grouped on the command line like `abc`.
- The inverse option for `bool` args can now be disabled.
- Fixed some issues related to linting.

- Fixed precedence of globals in `run` command. In particular, globals passed
via the command line have higher precedence than globals specified in the
`envs` section of the config file.
- Fixed a logic bug in `util.misc._merge_dicts()`.