imp(macros): add subcommand expr macro

[shua]

Was using some hacks to use both the macros and subcommands with hyphens in their names (example based on a cli tool I use a lot: https://concourse-ci.org/setting-pipelines.html)

clap_app!(fly =>
  (subcommand: clap_app! { @app (SubCommand::with_name("get-pipeline"))
    (alias: "gp")
    ...
  })
  (subcommand: SubCommand::with_name("list-pipelines").alias("lp"))
)

Added some parsing rules to allow alternative parsing of expr subcommand names similar to that used for (@arg ("non-ident-name"): --("non-ident-flag"):

clap_app!(fly =>
  (@subcommand ("get-pipeline") =>
    (alias: "gp")
    ...
  )
  (@subcommand ("list-pipelines") => (alias: "lp"))
)

and

clap_app!(@subcommand ("set-pipeline") => (alias: "sp"))

[kbknapp]

Could you rebase onto the latest master (which is now at v3-beta). Thanks!

[shua]

@kbknapp dunzo, waiting to see if I have received the appveyor+travis blessing.

[kbknapp]

@shua Sorry for the headache...there was some confusion due to my absence over the past few months...

v3-master "just now" became master in prep for a beta release. This PR was autoclosed when the v3-master branch went away. This will need one final rebase onto master 😟 Again, apologies

[shua]

as a heads up, travis checks are failing for rust v1.33.0 with

error[E0658]: use of unstable library feature 'iter_copied' (see issue #57127)
   --> /home/travis/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/toml-0.5.4/src/de.rs:435:30
    |
435 |                             .copied()
    |                              ^^^^^^
error[E0658]: use of unstable library feature 'iter_copied' (see issue #57127)
   --> /home/travis/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/toml-0.5.4/src/de.rs:575:22
    |
575 |                     .copied()
    |                      ^^^^^^
error: aborting due to 2 previous errors

(eg https://travis-ci.org/clap-rs/clap/jobs/604794320#L538 or https://travis-ci.org/clap-rs/clap/jobs/604736365#L535 )
and mine is no different

Success and ✔️ tests pass for other versions tested though (beta/nightly/stable)

[shua]

@kbknapp ^ I suspect master would not pass travis-ci checks right now due to some transient toml dependency using an unstable feature.

[CreepySkeleton]

@shua Perhaps you can restrict toml version on your end

[kbknapp]

I think we're going to just up the minimum supported Rust to 1.36 which should take care of it. See #1590 which should unblock this PR.