Supported spec versions moved from CLI tool to spdx package

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>
Initial validator code.

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>
Add spdx.ValueCreator type and start using it. Close #25

Fix: Update "SpecVersion" to "SPDXVersion" in tag/parser_test.go.

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>