Adds a plain rpm-prefix option, which doesn't prefix all files.

Sometimes, we like to make partially relocatable RPM packages, where usually some files in `/etc` are not prefixed. i.e.:
```
/etc/init.d/myapp
/etc/sysconfig/myapp
/etc/myapp/config
/work/myapp/bin
/work/myapp/lib
/work/myapp/var
...
```
If I package this (not using fpm) with `Prefix: /work/myapp` that part is relocatable, but everything in `/etc` stays put.

I was initially going to look at modifying `FPM::Package::Dir` since the other package types utilize its chdir/prefix implementation, but I wasn't sure how that would effect anything else.