add convert command for importing non-semantic versions

[eedwardsav]

spec for a new command

convert versions that don't fit semantic version into a semantic version compatible format

convert major to major.minor.patch

$ rerun semver: convert \
  --input_version 1
1.0.0

convert major.minor to major.minor.patch

$ rerun semver: convert \
  --input_version 1.5
1.5.0

convert major-rc to major.minor.patch-rc

$ rerun semver: convert \
  --input_version 1-rc5
1.0.0-rc5

convert major.minor-rc to major.minor.patch-rc

$ rerun semver: convert \
  --input_version 1.5-rc5
1.5.0-rc5