rpm: Add a option to insert raw tag statements

This can be useful to have requirements for different stages.

Examples:

    --rpm-tag 'Requires(post): /usr/sbin/alternatives'
    --rpm-tag 'Requires(preun): /usr/sbin/alternatives'
Merged pull request: #687: rpm: Add a option to insert raw Require statements

* pr/687:
  rpm: Add a option to insert raw tag statements