Implement flags for separator special handling

[AMDmi3]

Some version comparison schemes have special meaning for certain version separators, such as pre-release for tilda and post-release for plus (Debian), or pre-release for minus (full SemVer). We may introduce flags for supporting these, which may extend libversion usefulness and even allow to tune it to be completely compatible to e.g. Debian rules (#9).

Additionally consider:

• Handling separators as full-fledged version components, which would allow handling e.g ~~ properly.
• Handling invisible pre/post-release modifiers (may be useful for Repology).