Add open document option

[jeffa5]

Fixes #109

Adds the --open option to the build subcommand of the new v2 cli.

[codecov]

Codecov Report

Merging #733 (0d0c051) into master (21aaecb) will increase coverage by 0.00%.
The diff coverage is 70.58%.

@@           Coverage Diff           @@
##           master     #733   +/-   ##
=======================================
  Coverage   46.83%   46.83%           
=======================================
  Files         130      130           
  Lines       59683    59700   +17     
=======================================
+ Hits        27951    27963   +12     
- Misses      31732    31737    +5     

Impacted Files	Coverage Δ
src/document.rs	69.23% <69.23%> (+0.13%)	⬆️
src/bin/tectonic/v2cli.rs	78.04% <75.00%> (-0.44%)	⬇️

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[pkgw]

Thanks so much! This looks great and I just have a few comments.

1. It would be great to get some test coverage, although I can see how that might be tricky with the behavior that's being added here. Maybe the open crate has some test infrastructure that we can copy?
2. It would also be great to add the new option to the docs in docs/src/v2cli/build.md

Below I've also made a few very small comments.

[jeffa5]

I've added the docs and addressed the comments.

Regarding testing I've added one with the BROWSER environment variable set. This seems to work with not opening the document and passes the test but the open crate doesn't seem to have any tests either.

[pkgw]

Brilliant!

Now that I have awesome release automation I try to get contributions into releases quickly, but FYI it will take a little while as I work through the infrastructural changes that I've been pursuing.