rpm: Add a option to insert raw Require statements

[vStone]

This can be useful to have requirements for different stages.

Examples:

--rpm-add-require 'Requires(post): /usr/sbin/alternatives'
--rpm-add-require 'Requires(preun): /usr/sbin/alternatives'

[jordansissel]

hmm, it's not really an 'add-require' if you specify Requires in the value. The rpm book calls this a tag. What about --rpm-tag 'Requires.....' ?

Citation: http://www.rpm.org/max-rpm/s1-rpm-inside-tags.html

Looking at a spec file, the first thing you'll see are a number of lines, all following the same basic format:
something:something-else
The is known as a "tag", because it is used by RPM to name or tag some data.

[vStone]

I'll try to run a quick sed -i on the code and update the pull request later today.

[vStone]

Rebased + adjusted commit.

[jordansissel]

Thanks for your work revising this patch <3

Merged!