add a --version flag to otiopluginfo + otioconvert

[ssteinbach]

Adds a --version flag to otiopluginfo and otioconvert. This:

• prints out the __version__ string
• searches for pkg_resource installed plugins, and prints out their dist info (which includes their version numbers)

Question for the community: After seeing this I kind of wish there was a way to also burn the git commit hash into the repo some how. Not sure if thats useful, but it something we could potentially do when we burn the version numbers in.

Output looks like this:

> otioconvert --version
OpenTimelineIO version: 0.14.0.dev1
No pkg_resource plugins installed.

[codecov-io]

Codecov Report

Merging #880 (d919575) into master (2a33b58) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #880   +/-   ##
=======================================
  Coverage   84.36%   84.36%           
=======================================
  Files          74       74           
  Lines        3090     3090           
=======================================
  Hits         2607     2607           
  Misses        483      483           

Flag	Coverage Δ
unittests	84.36% <ø> (ø)

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[reinecke]

Just the one behavior question to make sure there isn't a missing exit, otherwise looks good!

[reinecke]

Is the intent to not exit after printing version info and continue printing the usual info?

[ssteinbach]

Correct. The thought was that this program is just dumping information about the installation anyway, so --version is an additive option and doesn't prevent printing everything else. Happy to change that if you think its too much of a break from expected behavior though.