Recommend, but don't require, plugins use semantic versioning on versions

[mbbroberg]

We moved to SemVer for the Snap framework. It would be valuable to have the same for each plugin. Could we recommend - in documentation and through logs/errors - that plugins do the same?

One place to do so would be upon loading a plugin:

$ snaptel plugin load /vagrant/plugins/snap-plugin-collector-perfevents
Warning loading plugin:
Plugin version does not follow semantic versioning syntax. Semantic versioning is not required, but it is recommended. Read the latest best practices for plugins at: http://pluginauthoring.snap-telemetry.io

Where pluginauthoring.snap-telemetry.io is a redirect to PLUGIN_AUTHORING.md for now and later could be external documentation (I just set this up).

[nanliu]

Linking issue with #276 and #1403. @mjbrender any objections if I collapse this with #1403?

[mbbroberg]

@nanliu I'm game for rearrangement as you clean up these issues. This issue is a particular implementation idea that seems dependent on the result of #1403 but could be solved independently. I'm good with however you want to roll up these pieces.