improve freeze command

[spoorcc]

Freeze command can be made more user friendly:

• allow for single project freezes, so user can provide project name.

• in git/SVN superprojects the manifest is under version control, freeze in place to preserve the comments and layout, instead of making a backup.

Existing behavior should be unchanged:

• features/freeze-projects.feature
• features/freeze-archive.feature

When implementing try to add manifest edit functionality to the dfetch.manifest.manifest.Manifest class.