Make it easier to push semver tags

[vito]

This would be a nice sugar on top of additional_tags - Docker has a pretty strong convention of pushing a version like 1.2.3 under tags 1, 1.2, and 1.2.3 (assuming it's the latest release):

put: my-image
params:
  semver_tags: version/version

Might want to make the level configurable; if it's a patch for an older version it makes sense to push 1.2 and 1.2.3 but not 1:

put: my-image
params:
  semver_minor_tags: version/version

(just a quick mockup, there may be a more intuitive way to configure this)

[vito]

done with v1.0 (#214 + #230)