V2 CLI: Add external commands

[rm-dr]

Implemented the feature proposed in #737.
Is it really that easy?

A question:
Should v2_main exit earlier if an external command is given?
We probably don't need to set up colors and customizations if we're running an external command.

[codecov]

Codecov Report

Attention: 27 lines in your changes are missing coverage. Please review.

Comparison is base (41168a9) 46.29% compared to head (11df05c) 46.20%.
Report is 44 commits behind head on master.

Files	Patch %	Lines
src/bin/tectonic/v2cli.rs	3.57%	27 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1103      +/-   ##
==========================================
- Coverage   46.29%   46.20%   -0.09%     
==========================================
  Files         171      171              
  Lines       65111    65099      -12     
==========================================
- Hits        30142    30078      -64     
- Misses      34969    35021      +52     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ColeFrench]

Cool! I think it would be useful to add tests for each target (unix and windows) with a dummy external subcommand. An addition to the documentation describing this feature should also be helpful.

Hm, I wonder if the help subcommand should also list external subcommands. Cargo has a list subcommand that lists both internal and external subcommands. More generally, maybe structopt/clap can integrate with external subcommands...

[pkgw]

Thanks! I'm going to close and reopen this to rerun the CI to check that this still builds cleanly against the latest master.

[pkgw]

Agreed that it would be good to add test coverage for this, although I admit that it might take a fair amount of work to implement for relatively small benefit. It might be helpful to see if Cargo has a clever, lightweight way for testing this kind of functionality.

One further ask that's a little bit more important to me would be updating the documentation somewhere to mention this capability. I'm not sure if there's a single best place in the docs to describe it, but ideally it should be described somewhere.

[pkgw]

The Azure Pipelines CI failures are spurious — a side effect of me merging another PR while this one was running. They should go away with the next fresh CI build.