Add post-run hook to support sending desktop notifications

[dnephin]

Thanks to @glenjamin for the idea.

Instead of publishing notifications directly I'd like to add support for executing a script that is passed all of the necessary details, and can handle any type of notification.

The script should be called with env vars which contain a summary of the run, and a json blob could be passed to stdin to provide full details about any errors.

Such a feature might look something like this:

export GOTESTSUM_POST_RUN_SCRIPT=./scripts/notify-test-results.sh
gotestsum

Env vars:

TEST_TOTAL=100
TEST_FAILED=2
TEST_SKIPPED=5
TEST_ERRORS=0

I'm not sure how to encode more details. The junixml format might work, but is probably not the easiest format to integrate. A custom JSON document might work but will be more involved to maintain.

Maybe env vars will work for the first version.

[glenjamin]

When I use these I tend to have my editor window focused, and my terminal running tests hidden - so ideally the notification should get enough data exposed to be able to include some details about the failure:

• files that failed to compile
• the compilation errors
• tests that failed
• files and line numbers of failed tests

[dnephin]

Started some work in #76 which I think would enable a basic notification.

File and line numbers is not something that is currently parsed. Test names are easy enough, but the file and line numbers are for the failed asserts/checks not for the tests cases themselves.

[glenjamin]

I had a little play around with this, had to switch to a shell exec to be able to handle multiple arguments to the hook command well:

4728f41

[dnephin]

Thanks for your work on this! I've merged the original PR which adds the new command line flag, and opened #116 for possible next steps.

I'll also open a PR now for adding it to the documentation in the next release.