This commit uses [clap](https://github.com/clap-rs/clap) to handle
command line arugments instead of a custom rolled solution. The benefit
that `clap` handles all validation as well as providing many "quality of
life" or UX improvements such as hidden alias, suggestions on typo,
auto-generated help, etc.

A previous branch of this project used
[structopt](https://github.com/TeXitoi/structopt) (which internally uses
`clap`). `structopt` makes heavy use of Rust's Custom Derive feature to
make the code less verbose and more ergonomic. `clap` is nearing it's v3
release, which pulls `structopt` into the mainline code base. This
commit does not use the v3 `structopt`-like feature-set since that
release is still in alpha. This meanas the code is more verbose (yet
just as performant), and could be changed to the `structopt`-like
features once `clap` v3 is officially released.

This commit also does not utilize some of `clap`'s other features like the
ability to generate [Shell Completion scripts](https://docs.rs/clap/2.33.0/clap/struct.App.html#method.gen_completions)
which could be added later if desired.

This commit is *nearly* a 1:1 translation from the original home rolled,
to using `clap`. I say "nearly" because `clap` adds many features on top
of what the home rolled solution provided "for free" as in with no
additional configuration. Those features include:

* Argument validation: `--routes`, `--dns-servers`, and `--port` all now
have built in validation. If someone enters an IP that isn't valid, or a
port that isn't valid an error is returned with a friendly message prior
to advancing into `gnirehtet` code. `--routes` also checks the CIDR for
validity as well.
* Auto generated help message
* Suggestions on typos (this feature could be turned off to reduce the
binary size, see below)
* [Hidden Command aliases](https://docs.rs/clap/2.33.0/clap/struct.App.html#method.alias); The following hidden aliases have been added for
commands:
  * `rt` which maps to `run`: I noticed `gnirehtet rt` used to be used for `gnirehtet run`
* Long options for arguments (i.e. `--port` as well as `-p`)
* [Hidden Argument aliases](https://docs.rs/clap/2.33.0/clap/struct.Arg.html#method.alias); the following hidden aliases have been added
for arguments (which provides help for common typos):
  * `--dns-server` -> `--dns` -> `--dns-servers`: All of these map to the same thing
  * `--route` -> `--routes`
* Long and Short help messages: `clap` allows one to specify a short
message for arguments and commands when `-h` is used, and display a
longer more comprehensive message when `--help` is used.
* Multiple ways to get help: `clap` provides `-h`|`--help` for the base
`gnirehtet` command, as well as all it's subcommands, i.e. `gnirehtet
run --help`. There is also a `help` subcommand `gnirehtet help` which
can be used alone or to display other commands, i.e. `gnirehtet help
run`. This helps because people assume various forms of help, and should
be able to find the help messages no matter how they originally try it.

Binary Size:

I see that binary size is a concern. This is the size after this commit:

```
❯ ls -l target/release/gnirehtet
.rwxrwxr-x 2.2M kevin 23 Feb 13:22 target/release/gnirehtet
❯ strip -g target/release/gnirehtet
❯ ls -l target/release/gnirehtet
.rwxrwxr-x 1.1M kevin 23 Feb 13:22 target/release/gnirehtet
❯ strip -s target/release/gnirehtet
❯ ls -l target/release/gnirehtet
.rwxrwxr-x 973k kevin 23 Feb 13:22 target/release/gnirehtet
```

So it does increase the binary size, but not dramatically. By further
turning off [`clap`'s default cargo
features](https://github.com/clap-rs/clap/#optional-dependencies--features),
the size may be able to be reduced even more. However, I also am a firm believer that these slight
increases are well worth the features they provide.

Relates to Genymobile#243

wip: clap