implement addpkg -f FILE functionality

[fwyzard]

This is on top of #37 and #38 .

Implement in git-cms-addpkg the functionality of "addpkg -f FILE" .

Note that FILE can contain any number of

• comments (lines starting with #)
• empty line (lines with only white spaces)
• subsystems (e.g. FWCore)
• packages (e.g. FWCore/Version)

Leading and trailing slashes are stripped, so these are valid, too:

• //FWCore/
• FWCore/Version///
• etc.

[fwyzard]

Edit: comments are now allowed after otherwise valid lines. So, this will be accepted and properly handled:

• FWCore # a whole subsystem
• FWCore/Version # a single package